Don't forget about wowza media server, it's not free but it seems it's a little less then adobe. however keep in mind, if you don't own the server with a good backbone service you will likely be renting space on any of the servers, adobe, red5, wowza and what ever else is now available. the rent space plus bandwidth is usually pretty much the same cost no matter which platform you use, just a couple things to keep in mind before you try all the learning curves that each bring with them. If you go with red5 you will need to download and install the dev tools required for coding / programming in java and you will still need one of the newer flash IDE's as well.
red5 uses the eclipse IDE, For instructions on how to install the eclipse plugin, please see the following documentation on the red5.org site:
wowza also uses their own version of eclipse as well in their own IDE called wowza IDE2
I have tried them all and I found that the support is excellent for wowza, you typically get straight forward answers in a timely manner, the other two mentioned not so much. aside from that it's all up to your skills and / or want to learn new ones.
it's all java based in one form or another anyway.
I'm not going to post adobe site, everyone should know how to get there by now.
I hope this helps someone.