Streaming Media servers are necessary for Live streaming to handle video bandwidth and in some cases mirroring or acting as a content delivery network (cdn) to prevent slow down of a site.
The Main software services to help your encode your streaming video online are as follows:
Adobe Flash Media Live Encoder.
Microsoft Expressions Encoder.
Many video encoders support different video codecs but generally can output a live flash video stream.
ManyCam (supports multiple streams)
Adobe/Microsoft/Real also offer 'high end' professional solutions for streaming media for large audiences at high definitions.