We just used Wowza, since they had an EC2 instance you could just boot up at a higher cost than a regular EC2 instance. The documentation is ok, not amazing, but getting it running is pretty trivial thanks to their examples.

You can use a developer's version of Wowza (the big limit is iPhone streaming is crippled).

A single license is like $1000, but you can instead pay by month ($60, IIRC).

Wowza also has a deal with Amazon, so you can run Wowza on EC2 and only pay by the hour (Wowza licensing included).