OneHost: The chat windows are resizeable (just drag to resize in the upper left corner). You can also pop the chats out into a separate window if you want them even bigger.

You can get an API Key at www.envolve.com. Just click the big green "Try it for free". It should be pretty straightforward. The "Settings" tab is in your vBulletin Admin Control Panel. It should be the top button on the sidebar on the left. The "Options" button will appear directly below that when you click on Settings. Here's what it looks like:
http://www.envolve.com/plugins/vBulletin/pic2.png

Hope that helps!

What sort of usage will this cause on the server using the free system?

It's a completely hosted chat solution, so it will have no impact on your server at all. All traffic is routed through the Envolve servers. This is true of all the version, including the free and paid versions.

Cheers Andrew for the Quick reply!

if a user log in my forum they able to see other friend online or not?

Of course! You should give it a try! I think you'll really like it. I'm new to the vBulletin community, but we've been working on this chat for more than a year-and-a-half. I think we've got something pretty slick here! If you want to play with it it's running live on our website.

@rajubd - We don't have a "friends list" at the moment. We actually show all the users who are active in chat to everyone. So yes they'll be able to see their friends, but they won't see only their friends. We've built this with a special focus on group conversations (though it does private 1-on-1 chats well too).

It is exactly what I was looking for, but unfortunately it hosted in your site .

So there was a long discussion thread here about other chat systems such as Comet Chat, Arrow Chat, and some others that you can install yourself. https://vborg.vbsupport.ru/showthread.php?t=176924

The Envolve chat is hosted because we can provide better service for large sites this way (apache + php doesn't do real-time stuff very well), and because this way we can update everyone immediately when we fix bugs / add features / etc. Did you try it out? Any feedback other than the hosting issue? Why is us hosting the service a negative for you?