I mentioned this over at I Waste The Buddha With My Crossbow yesterday, but this is one of the best resources I can share:
U.S. Army Field Manual Library
Aside from the obvious applications for warfare or modern games many of the manuals hold, you'll really want to check out 21-76, which is the Survival Guide. As Jeff Rients said, Gary Gygax once recommended having a Boy Scouts of America handbook and an Army Survival Guide. Tracking, traps, knots, terrain, survival methods--all things that can come up in a wilderness or dungeon crawl, and all items that can be fleshed out or given a new direction with a quick peek at the manuals now and then. Hope this is as useful to you as it has been for me!