These are pretty advanced nowadays. Here are some fun ones I’ve been meaning to link to on this blog for a while:
goosh.org—the unofficial Google shell
More seriously, these things are very useful so long as you remember to force https. I used to use shellinabox to get to mutt from libraries; it’s very impressive. It took me a while to find a decent browser SSH client but I’m now fairly confident that this is the best one out there.