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Termlib is what you are looking for I guess. It provides a "Terminal" object and you can extend it according to your requirement.
Demos could be found here
Examples: Anyterm, Ajaxterm
You could also use ssh if your firewall / site setup will allow it.
Or you could use vnc to get gui access, if you run a gui on your servers.
I think the best way to get access is either ssh or vnc. If you use Windows servers then remote desktop.
The main reason I could see for using a javascript based in browser UI would be if a firewall between you and the server won't allow ssh and you don't want to or can't configure vnc on the machines involved.
Finally, a really nice looking example of this kind of access on a web site is at... koding.com I saw these guys at the SFJS Meetup a few months back.