TVersity seems to get a lot of good press. I haven't tried it myself though.
Your other options would be (1) burning the AVI to a CD/DVD and playing it directly on the 360, or (2) accessing your PC directly from the 360 by using the "Videos" option on the "Media" blade (you'll get instructions on what you need to do after that). With the latter, you basically setup Media Player on the PC to share certain folders, and access the content directly from the 360. It's more convenient (and probably easier to setup) than streaming, but the supported file types and video encodings is far more limited.
Sony KDL-52EX700, Toshiba HD-D3, Motorola DCT-6416, Popcorn Hour A-110, XBOX 360 w/HD-DVD