Geocaching.com
is an outdoor treasure hunting game where you use your GPS to find hidden bounty, or geocaches. It had its heyday in the early 2000s when hand held GPS devices became more popular and readily available. Just the same, it's still great fun, and a cool excuse to combine hiking and technology.
Slackerpacker
This is a great site that aggregates hiking trails by state and by category. The site has information on everything from to geocaching. I often visit the site for its .
USGS: U.S Geological Survey
If you're planning a serious hike or backpack, you need topography maps. It used to be a lot harder to obtain specific mapping information, but it today's digital age, it's easy, and free! Created in 1879, the United States Geological Survey has cataloged all topography for the United States. You can search for and download , or for a couple bucks, you can order a paper version. And considering it's a government run organization, for me, the maps show up fairly fast (about a week).
Trail Journals
This site has reports from a growing number of people who hike some of the longest and most well known trails. From the Pacific Crest trail, to the Appalachian trail, this is a great resource for first hand accounts.
Natural Born Hikers
Some of my friends tell me that this site is need of a refresh, but in this complex web 2.0 environment, I find the site's simplicity refreshing. A large group of regular hikers send updates with firsthand knowledge and photographs of hiking trails listed for over a dozen countries.