You'll need something that gives you more than the basic EADGBE set. I've been using a Boss (Roland) TU-12H for a lot of years. It does the job very nicely, but its just a bit bulky. I recently got an IntelliTuner DCT-12L, which is about the size of a pack of cards.

Check out the pawn shops. I paid $20 for the Intellituner, instead of ~$80.