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