Codepad is an interesting site. It gives you the ability to post code in a text box, have the code run, and displays the code/output in return. It gives you a convenient URL you can point to later.
Here is a hello world program I wrote in C.
Here is a Python helloworld.
I found out about codepad from the authors post where he mentions how he was having problems scaling the site, and how he solved it using Amazon EC2.