Many people here have recommended the Yorkville YSM-1 monitors. YOu can find a pair new for ~$200 or so. I bought a pair from Daddy's Junky Music for $90, and they sound great. They have a good low frequency response and seem pretty even across the frequency range.
They're my first "real" monitors, so I don't have much to compare them to, but the price is right, and many people who own them say they are much better than anything else in that range.
Non-powered monitors, btw.