Crate's line of Taxi Battery Powered Amplifiers can be the answer when you need amplification but don't have access to a power outlet. I bought a TX15. It also comes in a larger TX50DB model.
The amp offers up to eight hours of battery power. Dual channels allow you to control two instruments or plug in an instrument and a mic (1/4"/XLR combo input jack). The 8" speaker does a pretty good job of delivering a clean, loud sound. The TX15 includes a 120V charger/adapter for standard operation and recharging the battery.
It's not super high fidelity but for the price (~$170), it does a surprisingly good job. Ideal for beach weddings and the like.
Here is a photo - in use at the Jersey shore last Saturday. I plugged my ukulele and mic directly into the TX15. It got the job done.
I owned the amp for about 11 months before discovering a problem with the battery not recharging. After a quick diagnostic over the phone with customer service, they shipped out a replacement. I was pleased by their service and decision to replace the amp.
I like my TX15, too. It does very well with guitar as well. I've been using my two Roland Cube Street amps a lot lately for all day beach jams. They run 15 hours at full power on a set of AA batteries and have almost as much acoustical output as the TX15, with the addition of effects seperate on both channels. Here's a past review of battery powered amps that I need to update: