It all depends on what your needs are...
- Wowza ($1k/license, probably the most popular)
- FMS ($1k/license, adobe charges arm and leg for support)
- Red5 (Works for live streaming, but doesn't support h264 files)
- Nginx RTMP Module (used it once for streaming files, don't know about live streams)
- C++ rtmp server (
http://www.rtmpd.com/, never used it)