I use FlashChat and integrated it with my UBB login. Only registered forum members can use it. Set up is simple and you have the choice of using it's own user registration or integrate it with your forums during the set up. I stuck it in an iframe to make it easier to place and control the size which means it can be put pretty much anywhere. Ironically I was just tooling around the support forums because I need to load it on a new server, if you go with FlashChat it looks like you want to avoid version 5 if possible. There seems to be a lot of chatter about bugs and a security issue that I did not see any indication they were all fixed. Go with version 4.X if available. Definitely worth $5 for the script.
