Me, I don't use too many tools outside of the game books. No apps, no special gimmicks. I wish I could get rid of the minis and battlemats, but my players would kill me (but we run gridless often enough). My only tool is my GM binder, which is filled with homebrewed random encounter charts for regions, my adventure notes, and my current favourite tool - the campaign calendar.
One of my players is secretly a lycanthrope/seawolf, and doesn't know it. Knowing when the full moon is occuring is quite useful. It helps for when I drop the quest to another player at a table who is trying to "solve the grisly murders in Bronze Shore" that seem to be on the rise....
Seriously, though- GM binder. Fill it with hirelings, villain stats, campaign information, your deities, maps, and whatever else. Record everything that could be important. Keep it (relatively!) organized. When you feel unable to run a game, open it up, and I guarantee you'll find an adventure hook if you keep that thing stocked.