Well
BitMit is definitely the biggest one out there, I heartily recommend it.
Unfortunately there is a big cloud over BitMit in the moment as to exactly who owns it, and who has control over escrowed funds. BitMit don't appear to be interested in trying to clear this up either. How can you trust an escrow service if you don't know who runs it?
This is discussed in a number of threads on bitcointalk, for example:
https://bitcointalk.org/index.php?topic=135366.0That's them off my list then.
Seems the only real place is to trade here and otc with individuals direct.
I have to say as a businessman my early impressions of bitcoin as a medium for doing business with isn't good seems safe to assume everyone is a crook at worse or unprofessional at best until proved otherwise.