I happen to have owned quite a few cheap 2.0 speakers that worked just fine. Here's a list, including places I know for sure you can find them (if you're in the US):
Creative Labs 2.1
- $20 at Wal-Mart. No headphone jack, but far more bass than the little 4" baby sub would suggest. Overall quality is good, but if you're looking for something without a sub . . .
Creative Labs 2.0
- $20 at CompUSA or most other large electronics stores. I have these, and while the bass is nothing to write home about, the treble is surprisingly crisp for a cheap set, and the midrange and volume are good.
Altec Lansing 2.0
- $30 at CompUSA, Wal-Mart, and other places. These have the just about the best sound quality you'll find from any two-piece system for less than $40 or $50. I have a pair, and like them quite a bit. Don't expect them to shake the walls, but they do have good wide-range response.
- $20 at CompUSA. I also have a pair of these - each has a 3" driver, and separate knobs for volume, bass, treble, and a button for a decent 3d effect and another for power. They take up a goodly bit of desktop space, but they get the job done. I'd recommend the Altecs over these, if you can stand to spend the extra $10.
There's also another option I know of, if you can find a CompUSA that still has them. For $25, you could have a pair of 20W wood-enclosure computer speakers, each with a 4" woofer and tweeter, rear-ported, with rear-mounted bass, treble, and volume controls. Better sound quality than any of these I've listed, but the volume knob on the back is annoying. Not a problem if you have volume buttons on your keyboard, though. I've had this pair for over a year, and I love 'em.