This wiki was started after a great discussion on NPN, read this post for more specific user experiences. Please feel free to edit this wiki to add other vendors or more details.
KEH - Has been around a long time and is highly trusted
MPB - Some users report good experiences, not as well known as KEH
Adorama Used - Trusted vendor
B&H Used - Another trusted name
LensAuthority - Sister company of Lensrentals who sells their used equipment 18-24 months of being in service
Canon USA Refurbished. You need to put refurbished in the search field and you can find them.
BorrowLenses - Quality gear from their rental fleet.
Fred Miranda Buy/Sell Forum - Requires paid membership and has a member rating system, great place to buy used gear from other members
Dgrin Flea Market - Smugmug’s forum, does not have a review system but seems to have many trusted
Ebay - may be a good source, depending on what you are looking for and how much effort you wish to spend doing homework. I wrote some pointers in this entry of the original post which I expanded a bit.