There are at least 6 different PHPBB MODs that give you 'chat' capability, including one from http://smartor.is-root.com
. Those aren't as flexible as the pay services might be, but they also do not have to use tricky means to "mine" your data for authentication (one I saw had some interesting ways to get information you might not want to share).
Chat was, at one time, the "most wanted" feature by users on one of our boards. We put it in. Activity was high for about two weeks, then disappeared. Users figured out that instant messaging wasn't as important to them, since the world-wide nature of the site (exchange students) meant that their friends might be fast asleep when they were wanting to chat...
When we did a forum update late last year, we didn't put chat back in... It took over a month for anyone to notice that the link had disappeared!