Update May 2009: Immigration to the U.S. has slowed considerably due to the recession and the loss of entry-level jobs in construction. Fewer people in the casual labor pool could result in more opportunities for those willing to take on the odd job.
You're out of cash. What do you do? Hit up Moms and Dads.
No, not your mom and dad, silly. Moms and Dads; those worn-out individuals with small children under five who barely remember what it's like to go to a restaurant and not spend the evening quelling tantrums and wiping up spilled water. There are a lot of part-time jobs you can get in order to turn a quick buck, but exercising the babysitting option is one of the most immediately lucrative. Supply and demand, you know.
I mean, babysitters earn upwards of $10 an hour these days. Offer to sit for friends' kids Friday or Saturday night. The parents will be only too happy to hand you cash when they return. One friend of mine used to leverage her evening take by mentioning casually that she'd be happy to spend the night with the kids for $100...which was always quickly obliged by the male half of the equation, if only so he can get eight hours of uninterrupted sleep.
Reliable babysitters are in high demand. You can almost name your price. Indeed, as a parent, I have been known to offer last-minute desperation bonuses for a sitter who'll show up on a moment's notice when the original sitter is a no-show. I gave one girl her regular pay plus a $50 tip, just to thank her for pinch-hitting at the last minute. (It was cheap compared to losing out on my $300 theater tickets).
You probably better have some experience with kids, either through raising your own or having had a lot of babysitting experience.
What if you're a young male, and nobody is likely to hire you to watch their kids? Offer to wash cars, mow lawns, weed backyards or haul crap out of the garage. Help folks move. Use your brawn, in other words. And state your hourly rate up front and insist on cash.
Other quick cash options? House-sit for friends of friends (friends by definition, will expect you to feed their cat and take in their mail for free. People you only know by association won't). Walk dogs. Haul recycling. If you're handy with a musical instrument, go to a tourist spot and busk.
Mostly, use your brain. There are a lot of options for those cash-less souls who're willing to work.