This is the one I am using. http://www.providesupport.com
It comes with a desktop app and a web app for remote assistance.
Since I installed it, I have converted about 90% enquiries into a sale and it only costs $99 / year.
This is who we use. Very reliable. We get about 2 people a day using it for very simple silly questions. I think its really good for people who have a little query and cant be bothered calling (worried they'll have to go into the ins and outs). The other nice thing is you can LIVE monitor who is on your site. You see the IP address, country, pages they are looking at in real time.
We have been using 'Live Chat' on one of our production sites for some 6 months now. As mentioned in other comments you need to change your frame of mind to 'Live Chat' but once done the benefits are enormous we have noticed a noticeable difference on the bottom line.
We are using 'Provide Support' which integrates well into Actinic V9.
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