Anyterm
It's now 11 years since I wrote Anyterm, the first terminal-on-a-web-page:
http://anyterm.org/
There are some demos there, but you'll probably crash it if you all try at once.
It's open-source (GPL) and still maintained - I just added IPv6 support a few days ago.
For those of you asking "why", the main answer is so that people who find themselves behind an http-only firewall can still do command-line stuff on external machines.
My implementation was largely constrained by the technology available at the time (and the code still has lots of work-arounds for hopefully long-since-fixed browser bugs). It should be much easier to do today if you start from scratch.