I use PhpLive (www.phplivesupport.com) and they seem to work great. There is a large up front cost, but after the first year, its only 70 for each additional year. There have been some bugs upon new releases, but they were fixed promptly.
OOO! I just looked at their website right now, and they are having a 15% off sale for today only. (Trust me, they have never had a sale before) If you need a chat software, buy this today.
I'd second that... PHPLiveSupport has worked well for us. It is a nice solution and you can customize it if you get the source code option for a one-time fee.
If you are on a tighter budget, their monthly hosted option is also very cost effective.
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