I have been going through all these myself. Have looked at:
Envolve
Skysa
123WebMessenger
Flashcoms Site Messenger
CometChat
ArrowChat
I have also looked at other "Social Bars" that have chat like:
Wibiya
Bumpin
Meebo
It has been an exhaustive couple of weeks going through them all and getting users to comment, had a poll, tested on live site etc
The question comes down to whether you want just an instant chat and chat room or a whole "Social Bar" with instant and group chat...and what type of login do you want, integrated with your site or through a Social Media Identity like Facebook etc.
The question is whether you want a free solution or a paid solution...and if paid, do you want a "one off" payment or monthly payment.
So it comes down to what suits your site, number of users, cost and whether you want instant chat only or a Social Bar with instant chat.
At the moment we have started using Envolve "Free" but during testing we found the 25 user chatting limit was hit a couple of times so we will just monitor that and go from there. We already had 123FlashChat for group chat so we have turned the group chat in Envolve off. For "Socialising" our site we are also using the Conduit Tool Bar so users can download and install our tool bar for their browser and use the 123FlashChat no matter where they are on the internet however it has our own links on it so users can come back to our site by clicking our icon on the bar. The tool bar is also something we will be developing further with connections to our site.
The "final" thing that I will be looking at is the whole social thing on our site. What I am going to explore is a menu item "Socialise" that could have a whole range of social media tools for users like tweeting, facebook status updates, linkin updates, our chat room, a widegt of our YouTube channel and more, all from a drop down menu and pop out widget windows.
Hope this helps others