Geocaching Timeline

1854
What is considered to be the first Letterbox is placed in Dartmoor, England.
1976
Tom Gant creates a map of the 15 letterboxes then in existance, popularizing the hobby.
Feb 22, 1978
The first GPS satelite, Navstar 1, is launched.
April 1998
Smithsonian Magazine publishes an article on Letterboxing, creating worldwide awareness and interest in a hobby that had until that time been mostly limited to SW England.
May 1, 2000
Selective Availability (SA), a scrambling feature that limited the accuracy of civilian GPS, is turned off by order of President Clinton
May 3, 2000
Dave Ulmer hides a "geostash": a 5 gallon bucket full of trinkets in rural Oregon. He makes a post on Usenet group sci.geo.satellite-nav announcing the placement and its GPS coordinates. This is recognized as the first geocache.
May 7, 2000
Mike Teague places GPS Stash #2 and #3, both on Mount Saint Helens.
May 8, 2000
Mike Teague sets up the GPS Stash Hunt Homepage to collect the location of geostashes around the world.
May 9-13, 2000
Inspired by Dave Ulmer, geostashes are placed in California, Kansas, and Illinois.
May 11, 2000
Mingo is published in Missouri. The 7th published geocache, it becomes the oldest active geocache in the world in 2004 and remains active as of 2011.
May 12, 2000
The first geocache outside the US is placed by Peter McKellar near Rotorua, New Zealand.
May 15, 2000
The first geocaching email discussion list - Global Positioning Stash Hunt (GPSH) - is started by James Coburn on eGroups (now Yahoo Groups). The list is still in existance, although traffic decreased heavily after 2001.
May 26, 2000
Dave Ulmer points out the negative conotations of "stash" and asks for suggestions on what to call this new hobby.
May 30, 2000
Matt Stum suggests "geocaching" as the new name. Dave Ulmer immediately approves.
June 3, 2000
The first geocache in Europe is placed by Chris O'Byrne in Bray Head, County Wicklow, Ireland.
June 15, 2000
The first Virtual Cache is published (the type would be retroactively added when it was created). Other caches would be changed to Virtuals after the container went missing, but this was the first geocache to start out intentionally containerless.
July 2000
Jeremy Irish, then a Seattle web developer, first learns of geocaching while researching GPS technology.
Aug 1, 2000
The first Multicache is published (in Missouri). An older cache (in Georgia) was published in June 2000 as a Traditional and modified in 2003 after adoption by a new owner into a Multicache.
Sep 2, 2000
Jeremy Irish announces he has registered the domain name geocaching.com and sets up a new geocaching website. The website debuts with 75 total geocaching listings worldwide.
Sep 6, 2000
Mike Teague announces Jeremy Irish and geocaching.com will take over geocache listings from his GPS Stash Hunt Homepage, citing lack of time on his part and better functionality of the new site.
Sep 25, 2000
Slashdot runs an article about geocaching for the first time.
Sep 30, 2000
The first Unknown/Mystery cache is published. It was originally a Moving Cache, but was later changed to a Puzzle Cache in a fixed location.
Oct 2000
The New York Times becomes the first major newspaper with an article about geocaching.
Oct 8, 2000
The 100th geocache is published (in Melbourne, Australia).
Dec 19, 2000
Jeremy Irish first pitches the idea of geocaching clothing.
Jan 15, 2001
The first Letterbox Hybrid cache is published.
Jan 26, 2001
Jeremy Irish files a request to trademark "geocaching". The request would be dropped by July 2001.
March 2001
Navicache.com is started by Quinn Stone as a regional caching site.
Mar 24, 2001
The first Event Cache (then called a geo-gathering or geocaching get-together) is held in Austin, Texas. It technically recieved the Event type retroactively.
May 2001
Ed Hall's website Buxley's Geocaching Waypoint gets in a legal dispute with Jeremy Irish that lasts until 2006.
May 7, 2001
Geocaching.com adds seperate cache types. The original types were Traditional, Multicache, Letterbox Hybrid, Virtual, Unknown, and Event.
June 2001
Navicache gains popularity thanks to a publicized legal dispute with Jeremy Irish about the use of the term "geocaching".
June 22, 2001
GeocachingWorldwide.com is started by Jeremy Hurst, but shuts down by the end of the year.
Aug 30, 2001
Jeremy Irish releases the first TravelBug, Deadly Duck: Envy.
Sep 1, 2001
The first GeoCoin is activiated by Jon Stanley aka Moun10bike, who is credited with the idea. The coin remains in his personal collection to this day.
Sep 12, 2001
The first Locationless/Reverse cache is published (it would retroactively recieved that cache type when it was introduced in 2002).
Sep 30, 2001
The second GeoCoin activated (also by Moun10bike) is the first geocoin released in a geocache.
Oct 11, 2001
The first webcam cache is published.
Apr 24, 2002
Locationless/Reverse caches are introduced as a seperate cache type.
Feb 2002
Geocaching recieves a dedicated Usenet group: alt.rec.geocaching
June 2002
GeoGamer.com briefly debuts as a commercial geocaching site.
Apr 2003
CITO is added as a distinct event type, starting with 67 cleanup events held on or around Earth Day 2003.
June 21, 2003
The first GeoWoodstock event is held in Louisville, Kentucky.
2004
The letterboxing website AtlasQuest debuts.
Jan 1, 2004
The first Earthcache is published. A few older caches were later changed to type Earthcache.
Oct 2004
Terracaching debuts as a geocaching website geared toward serious cachers and higher quality caches.
Oct 10, 2004
The first Earthcache is published on geocaching.com
2005
Waymarking.com opens. All Locationaless/Reverse caches on Geocaching.com are archived and future publication of Virtual and Webcam caches prohibited.
May 27, 2006
GeoWoodstock 4 becomes the first Mega Event. The event type is retroactively changed after attendance surpasses 500.
2006
The Opencaching network begins, linking a series of country-specific geocache sites in Europe and later elsewhere in the world.
2007
The first geocaching World Wide Flash Mob (WWFM) takes place, organized by Team Podcacher.
June 2, 2007
The first Groundspeak GPS Adventure Maze opens at the Minnetrista in Muncie, Indiana.
Dec 31, 2007
Geocaching.com reaches 500,000 active geocaches.
Jan 8, 2008
The first Wherigo cache is published.
2009
Having become the first geocacher to log 25,000 Finds, CCCooperAgency retires from geocaching.
Apr 4, 2009
Additional Logging Requirements (ALR) are banned from all physical caches. The only exception is the challenge requirements of Challenge Caches.
Mar 8, 2010
Geocaching.com reaches 1 million active geocaches.
May 1, 2010
Geocaching.com celebrates 10 years of geocaching with a series of worldwide events.
May 25, 2010
The ET Highway power trail in Nevada is published for the first time. It would be archived in March 2011 at the request of the NVDOT.
late 2010
The Opencaching network expands to the US with the debut of Opencaching.us
Oct 10, 2010
Geocaching.com organizes a series of "10-10-10" events around the world and successfully sets a new site record for most users logging at least one find in a single day (over 70,000).
Dec 7, 2010
After weeks of teasers, Garmin debuts Opencaching.com as a new geocaching site.
2010
Alamogul (10,551) and Bobcam (10,568) become the first geocachers to log more than 10,000 Finds in a single year.
Mar 12, 2011
Alamogul becomes the first geocacher to log 50,000 Finds.
July 2011
Munzees debut, with a GPS game where finders find and scan QR codes with a smartphone app instead of signing a logsheet.
July 2011
The ET Highway power trail returns with some changes. Businesses in the area (such as Rachel, NV) successfully lobbied for the caches to be allowed because of the boost to the local economy provided by cachers.
2011
Alamogul becomes the first geocacher to log 60,000 Finds.
2011
Team Snook become the first geocachers to log more than 20,000 Finds in a single year.
June 30, 2012
Project: GeoGames (GC32012) becomes the first geocaching event with over 3,000 Attended logs.
Sep 2, 2012
1st International Earthcache Event is held in Maine and goes mega.
2012
Alamogul becomes the first geocacher to log 80,000 Finds and the second cacher to log more than 20,000 Finds in a single year
Feb 28, 2013
Geocaching.com reaches 2 million active geocaches with the publication of "Overwatch #1" (GC46N4E).
July 10, 2013
Bobcam passes Team Snook for second most lifetime Finds.
July 13, 2013
International Geocaching Get Outdoors Day
Aug 2013
"31 Days of Geocaching". Groundspeak awards a Souvenir for finding a cache on each day of the month.
Nov 6-7, 2013
Geocaching In Space. More than 1100 events in over 30 countries are held to celebrate the launch of a Soyuz rocket taking Rick Mastracchio (AstroRM) to the International Space Station. He brought with him a special TB and was FTF on the ISS geocache placed by Richard Garriott (LordBritish) in 2008.
May 2014
Alamogul logs his 100,000th Find at GeoWoodstock XII.
Aug 16, 2014
Munich, Germany hosts the first Giga Event.

Sources:
AtlasQuest's History Of Letterboxing
World's Oldest 100 Geocaches
GPS Games - The History Of Geocaching
Geocaching.com - The History Of Geocaching
Groundspeak Weekly Mailer - May 4, 2011
Groundspeak's Bookmark List Of The First 100 Geocaches
Isonzo Karst's Bookmark List Of First Geocaches By Cache Type
Cache Crazy
A Short History of Geocaching: May 2000