Like any other service you can run a radius server in the background with lots of other stuff, but I suppose it's best policy to setup a dedicated server just for this, reduces the chances to get hacked.
There are about 40 billion different radius-type things, both free, Free, and commercial....
Don't know much beyond that. Do a google search for radius and you get 2 free software implimentations of it on the first 2 results, read thru those docs and it should answer most of your questions. Even if you don't use them. That's what I figure anyways. What the hell do I know? not much.