ClickDesk - Expensive, but the only live support system that I've found that will work with Google Talk
One of the best live chat systems out there - if you can afford the fees.
Setup this system for a client who's very happy with it. Get's plenty of leads, so it's worth his money.
Wasn't impressed with the support response to the Java errrors after WP3.5 - and it took the ages to update the plugin. Only way to fix the thing was backdate to WP3.4.2 - Looks like brightonseo and others had a similar experience - time frame was WP3.5