For JavaScript (and HTML/CSS), including actual execution for live testing of the code (in your own browser, of course, but not in IE6), there's the brilliant JS Bin.
Be sure to watch the short introduction videos. Once you saved your code to a public URL it cannot be changed, but others can save their changes and then get a brand new URL they can paste in a forum or on sites like Super User.
Minor annoyance: it embeds Google Analytics tracking code (but only when running from the bare URL), which may interfere in some rare cases (like when trying to figure out if Is $(document).ready() also CSS ready?).