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[Vanilla]

i don't see any that are user vs. user IM, there all chat rooms

As I mentioned in my earlier post, these weren't "final" solutions, you were supposed to build your own script, but borrow some of the code from these. On second thought, I just came across this, it might be easier to pick out the parts you need from a framework:
http://ajaxim.com/

If you want anything more specific, I guess you'll have to either pay for it, or build it on your own.