Caching
Geocaching.com - The largest and most successful geocache listing site on the planet. With well over 100,000 geocaches hidden all over the planet, and a superb website, this is a great place to start your caching.
Navicache.com - An alternative to geocaching.com that hasn't really found its niche yet; it is largely defined by not being geocaching.com. The potential for this site is marred by the fact that many/most caches listed with navicache.com are also listed on geocaching.com, so most people go to the bigger site.
Terracaching.com - A new site with an interesting model for membership and listing caches. TC.com also has a unique point system for ranking players and caches so as to (hopefully) insure high quality of both.
Movingcache.com - A new site focussing on moving caches, with the placer/owner keeping lots of control over each game (cache), as regards points earner for moving the cache, rules governing the range of the cache, how far it should be moved each time, the terrain and difficulty of each new hide, among other variables.
GPSGames.org - A neat site with a host of GPS games including Geodashing, Geogolf, Minutewar, Geopoker, and their own version of geocaching.
Earthcache.org - geology related virtual caches donen in partnership with geocaching.com.
Caching Related Links
"Today's Cacher" - a superb monthly dealing with the world of geocaching and GPS hobbies.
Geocacher University - a great site with tons of information and useful links for new geocachers.
Buxley's Waypoint - an awesome mapping site, caching and dashing links all over the world.
Handicaching - a site focussing on cache hunting and placing for people with disabilities.
The Geocachers' Creed - a collaborative effort to outline a code of geocaching conduct
Geocaching News on Google - up to the minute news from around the world having to do with geocaching.
EasyGPS (GPX/LOC) - A free application for managing both LOC and GPX file types.
Geocaching Swiss Army Knife (GSAK) - All in one application that allows you to manage GPX/LOC files, upload to your GPS, and convert to various other formats. Uses GPSBabel as the back end.
CacheMate - is a database for tracking GPS cache hunts for Palm users.
Amazon.com's GPS Center - a good place to compare and buy GPS units.
CheaperThanDirt.com - a great place to buy cheap ammo-cans and other caching supplies.
Campmor - a nice outdoor-gear store with a little bit of everything.
Oriental Trading Company - a fun place to shop for trade items.