Sedo is one I know of.
Check the following thread as well:
Places to sell domain names and websites? [webmasterworld.com]
[edited by: Habtom at 1:57 pm (utc) on Aug. 8, 2007]
If you want to discuss the potential value of your domain there are many forums that allow you to hold such discussions, but not this one. The valuations are some help but not much.
If you have a buyer then there are many companies that offer domain sales escrow services. For example, Escrow.com, Sedo.com, Moniker.com, Afternic.com and others.
The Domain Forum discusses domain name related issues but, for the most part, we avoid discussing specific domains or allowing the forum to be used as a place to endorse specific companies or specific software. We'd rather the discussion focus on "how to choose a company" instead of "what company to chose".
Since the 2 threads already linked to in this thread do a good job of answering your question - and since the posts that have followed appear more interested in endorsements than explaining "how to" - I've concluded that this thread has run its course and I'm now locking it before any more "Use this (company, forum, what)" posts appear.
When it comes to selling domains this is my constant advice: Good domain names sell themselves. Marketable domains naturally attract interest and buyers. If no one is contacting you about your domain chances are pretty good that no one sees a need to own it at any price.