I have used generators, Uninterruptable Power Supplies (UPSs), and other battery-powered solutions. None of them are particularly cheap, and depending on how much power you need, not all of them are suitable.
You need a pure sine wave output and your solution needs to deliver enough amperage to handle the current draw of your lights. Honda makes a series of generators (the EU series) that are clean, quiet, small, and perfectly suited to the task of powering strobes. They start around $700 for a unit, (EU1000i) that will supply 900W @ 7.5 Amps. That's enough for most lights up to 600 W/S. However, you're looking at about 30 lbs. and up.
You can also use an appropriately sized UPS. However, these end up costing just as much, if not more than, a photo-dedicated battery solution. In order to handle the current draw of two 300 W/S monolights, you need a 2000 VA UPS. Something in this range will run you over $1200 and weigh somewhere in the neighborhood of 70 lbs.
A poster above mentioned the Tronix Explorer 1200. It's a reasonable choice as long as you don't intend to use it with lights that exceed 600 W/S. There's a brief article here that gives it a decent review: http://www.robgalbraith.com/bins/con...id=7-7883-7908
Also note that according to the unit's compatibility chart: http://www.innovatronix.com/cgi-bin/...g/compatib.asp
, the unit might not be compatible with some strobes that have digital output control and displays.
Among the photo-specific solutions, the AlienBees Vagabond 300 is reasonable, at ~$500, and performs very decently with 600 W/S worth of strobes. It's a little on the bulky side, though certainly easier to lug around than a generator or UPS. Other solutions, like the Dynalite XP1100 will handle power packs or monolights up to 1100 W/S, but start to get expensive.