I would recommend the "NOVOTEL" in Brussels. Not expensive and pretty basic. Another pro is that it's in the very center of Brussels. However, I surfed to the page that Bart Couvreur mentioned:
http://www.fosdem.org/2007/practical/accomodation . There is a hotel: "CHAB: Centre Van Gogh" which seems to have a direct tramway connection to the FOSDEM area. I checked also their website and it is a + that you can individually book online for one or more attendees. In that point of view, I think last mentioned hotel is a good choice. Feel free to surf to their website:
http://www.chab.be/ . If you have other remarks or questions, don't hesitate to contact me.
Kind regards,
Bart De Soete
Fedora Ambassador - Belgium