If you have a receiver to plug/connect your sound card into:
Well right now, there's all this talk on other boards about the Michaura M55 sold at ubid.com
It's supposed to be damn near equivalent to the Axiom m22--a speaker that's rated to compete with $3,000 monitors. It's actually made by the same company.
They are going for less than $100 on ubid, but now are only coming in black, not cherry as shown in the product picture. It seems to be forward sounding and detailed. The tweeter runs the edge of bright with some recordings. Many happy campers with this one (I just bought a pair for a bathroom setup--just kidding!
I did buy one, though, to try--could......n...o...t....resist!! Should be here soon.)
============================================ If you want a self-powered unit:
The Aura 20/40. Go to Cnet.com for review and prices. Very highly rated. You probably can get it easily for under $40. Do a google.com search with this unit model to get other retailers who sell it.
I had this unit for 4 years and had great sound--easily should have gone for MUCH MUCH more--it's that good! I liked it so much, I even returned the Diva M200 (an audiophile recommended computer speaker), because it sounded bloated & uncrisp compared to my Auras. The Aura sounds great with MP3 and goes loud--great for rocking! It's very forgiving with bad MP3 recordings--that's a VERY good thing!! Other speakers will show the compression in MP3--that's a bad thing....a very bad thing for MP3. The thumping sub will get your neighbors to come over. It happened to me several times in different well-insulated digs in different cities.....and I don't even blast mine!
If you go with the Michauras or any other speaker, wait on the sub. For under $100, you'll get a pretty untight performing sub--put your ear next to one at Best Buy and see what I mean. My recommendation is to hold off on the sub until you have some scratch to buy something nice on Ebay.