JavaChat and an IRC channel is great (and free) for the chat rooms. It gives basic users the ability to chat from a web page, and more advanced users can download their favorite IRC client to connect to your chat room. It does require that you join and setup a channel on your favorite IRC server first.

As for instant messaging, Jabber is an open standard that has a large following already.

Honestly, though, your best bet is to just give your users a way to connect to each other with their existing IM solutions. You'll have a hard time convincing and then educating your users to download, install, and configure software to use for your service.

Most community software offers both private messaging (which is personal, but less instantaneous than an IM) and the ability for users to register their existing IM usernames with the community. This usually involves a field they fill out in their profile which leaves a link by their name whenever they communicate with other users or when other users view their profile. It'll be easier for you to convince your users to fill out this profile field than it will be to get them to sign up for a new service.

Thanks for the input, grey259. I'm wanting to install a service (two separate services, I believe) like on another site I know. The instant messenger service there doesn't involve the user downloading anything -- just on the user's profile page, you can click "IM", and automatically IM them. I'm looking for something like that. He has a free (userplane) chat room on there, but his IM service is for a fee. I'm having a tough time finding an IM/chat service combined.

[edited by: rogerd at 6:58 pm (utc) on Dec. 4, 2007]

I've been looking at Jabber myself which is a open source IM service that has gateways for the other major services. There's a few open source server software packages available, most allow for either personal chat or chatroom .