Other than the iPhone 4's onboard Maps and Compass apps, I've also got Here I Am (not useful without GPS), iTopoMaps (less useful without GPS but not useless), myLite (not useful without a flash), wikiHow: How to and DIY Survival Kit, iMedJet, the paid version of the Ultimate SAS Survival Guide, and the US Army Ranger Handbook.

I think all of them were free other than the Ultimate SAS Survival Guide. I'm a bit of a packrat when it comes to books and apps, with reference apps triggering both of my packrat reflexes.