FriendPaste is neat, but it's not as daring as letting people run a bunch of different languages on your server like CodePad does. I imagine one reason it's not open source is that they're relying partly on security through obscurity. I would be if I were running such a site.
BTW, I did check it out more in depth, and its python interpreter runs "import datetime" and "print datetime.datetime.now()" fine, but gives an error than an md5 library is missing when you try to run "import urllib2". So it supports some standard modules but not all.
Oh, nevermind, it's a #nosql site. I withdraw my argument.