First the official site:
Geocaching.com It’s free and easy to take part, with pages and pages of help and fun.
Navicache.com Similar to but smaller than Geocaching.com, but thousands of caches.
Earthcache.org Linked to Geocaching.com, but has a twist: caches provide a learning
experience about earth science.
GPSinformation.net Has more web links to every aspect of geocaching and GPS than
anywhre else I know. Worth looking at.