The three sites mentioned above have had great success, despite having domains that aren’t spelled correctly. The key is to be obvious. Make the misspelling noticeable, so people can tell you did it on purpose. By having a domain like this, you can have a short name that people will remember.

Digg is a great example. It’s only four letters, and that extra g really adds something. It’s effortless to remember the extra g, and you find yourself adding the extra letter in when you don’t need it. (I keep typing “digg” instead of “dig” now. Thanks a lot, Kevin Rose! You’ve etched your brand name into my subconscious.)