RubyIZUMI is an open source RTMP Server for Flash Video/Audio Streaming. It is written by (Pure) Ruby and it supports to broadcast MP4(H.264 + AAC).

UPDATE: In April, Adobe seems changed RTMP handshake method in their new Flash Player 9,0,124,0. So we can’t play a H.264 stream with RubyIZUMI and the latest Flash Player 9,0,124,0. At May 2, the guy Yaacov gave me a patch to fix this problem. (Thx Yaacov!) We can now play with the latest Flash Player. Use RubyIZUMI version 0.10 or later.

See more on the project page rubyizumi.