I bought a couple of Acer Revo nettops a while back. About $200-250, low power, quiet. They came with Windows XP installed, but newer versions of this box (which were a bit pricier) came with Windows 7 (and more RAM). They seemed to be very popular, yet, for some reason, Acer stopped making them at the cheaper end of the scale. I have to wonder if they weren't cannibalizing sales of their other desktops and there was probably less profit margin on these. The newest versions seem to start at around $330.
There are other manufacturers who make similar models (see ZOTAC ZBOX and ASUS EeeBox), some of which can be had for $200-$250, but these seem to be bare-bones models with no OS or RAM, so you'd have to add your own.
Update: If you check newegg.com, you can find a Lenovo for $300 with 320GB HD, 2GB of RAM, and Win 7. It even comes with one of their neat-looking mini keyboard/trackball remotes (I bought one of these keyboards a while back and decided it wasn't my cup of tea). Or, if you have an extra copy of Windows lying around, you can probably save a few bucks by going with the ASUS they have for $185 (no HD, RAM, or OS).