Well you've got the www.rentacoder.com type sites. www.odesk.com is another. And then you've got whatever you can dig up on your own through independent consulting. Just be sure not to break any agreements with the startup with respect to IP, competition, etc.

And then there's always waiting tables. A bit of a shot to the ego perhaps, but that'd be a good break from coding perhaps. If it's good enough for every budding actor/screenwriter/director in LA... :)