You want something that you can re-charge by plugging it into the wall. The Line-6 and the Crate are both very cool, lots of effects, but they eat the batteries.

The Fender Amp-Can rocks, though it's on the Trebly side. I use an Alesis Transactive Mobile. It's 20 watts, will hold a charge for 12 hours. You can run an Ipod or tape player through it, and it will charge your Ipod while it's being used.   

But, I used to have a Pignose Hog 30, and loved the sound. Big, and warm.   The Battery only lasted for 3 plus hours though and the skin was on the soft side - not as tough as street gear should be.