I have a set of Bose Companion 2 speakers.
Companion® 2 Speaker System
I really like them. They sound great, nice and clear even if you turn the bass up a little. They're not the smallest speakers in the world but still compact enough to fit on the desk.
They're pretty affordable too at about $100 a set. I got mine for $89.
If you want a little more bass they have a Companion 3 setup as well.
It has two really small satelites and an Acoustimass.
A little more expensive at $250 a pop.