The PS3 Media Server is freeware and has one nice feature, it converts every known format before sending it to the PS3. So you can also watch other standards as DivX, it's great to watch HD material, which often comes in mkv format. I didn't test it so far, but Zyriax skipped Twonkey Media Server now, and only uses this one, he told me that it's working fine.
http://ps3mediaserver.blogspot.com/
