You can't get a better deal on speakers than buying used today.
I'm an Infinity fan, and a few of the vintage little bookshelf Infinities have a legendary reputation today. There's the Modulus, which is normally paired with a servo subwoofer (but is still good without it, just less low bass), and the 1978 Infinitesimal, which features a dual voice coil woofer that goes astonishingly low with authority. The Kappa 5 performs similarly as well. They all have Infinity's renowned EMIT tweeters, which when amped correctly will be incredibly detailed and smooth at the same time.
Now if you've got some room to spare on or near your desk, I would go for no less than a pair of Klipsch Heresys. Seriously. Heck, you could probably power them with a desktop headphone amp. Realistically, get an 8-15 Watt little tube amp for them. If you get lucky you might find a pair for a steal at a secondhand store, but more often they're found for $300-$500 from knowledgeable sellers. If you've got the room, no bookshelf speaker or monitor could even compete - assuming you want something fun to listen to and rock out to - not a perfectly neutral, bland monitor.
Anyway, whatever you are looking for you should definitely check out Audiogon. The search tool is your friend, especially the price/category one. Craigslist is an excellent place to search as well. Don't be afraid to drive to pick up a good deal on the right pair of speakers either. I drove eight hours to pick up my Kappa 8's, and over twenty to pick up my Renaissance 90's (both floorstanding Infinity speakers). I don't regret either trip at all.