I came across this site a while ago.
What's your favorite Dungeons and Dragons resource? Categories: Dungeon Master Player Lore World building
DonJon is probably the best resource I've come across. I wish they put out an app too. I know I can just use the web interface, but an app could potentially cache some things locally.
Speaking of apps if you are so inclined (and want to use Pathfinder) you should give this a shot: https://play.google.com/store/apps/details?id=org.evilsoft.pathfinder.reference