Lloyd Alvarez recently released his BG Renderer script for After Effects 7 which will work with both Mac and Windows. BG Renderer is a handy script for managing background rendering tasks including launching multiple instances if you have multiple CPU’s. Lloyd’s put a lot of work into this script and it shows.
If you’re looking for commercial software to get more bang for your rendering buck, take a look at Nucleo and Nucleo Pro from GridIron software.