Is there a good SE site that is only about refactoring?
I know there are sites like this, but SO has the largest audience of programmers so a section where you can just post a code snippet and people can give advice on refactoring it.
For anyone currently browsing this question (October 2011), this currently exists at codereview.stackexchange.com (beta status).
Stack Overflow could be used to do that. In fact, I've used it for that in the past.
I'd recommend a tag like
refactor-my-code but either would work.
The 'question' part would be
How can I refactor this code for X
You undoubtedly seem to be looking for RefactorMyCode.com. This is a relatively new yet well-established site for anyone to come along and propose improvements/refactorings to your code, in a vaguely collaborative/wiki-like way.
As an addendum to the other answers recommending SE Code Review it should be noted that only real, fully working code is accepted there. Also you should describe what should be refactored in particular, and maybe add some description of your attempts.
Another option to ask for such from a more theoretical background (using a hypothetical code example) would be to ask at SE Software Engineering, but as well you should show some effort about your ideas, research and attempts.
I am not sure, but I think you are looking for codereview