Auctions

Afternic.com
Domain name auction site with variety of services like domain news, appraisals, and chat. Recently acquired by Register.com.
http://www.afternic.com/

Auction2.net
UK based site that mostly caters buying and selling of domains with .co.uk and .com extensions.
http://auction2.net/

TopComs
Offering a selection of .com domains for auction, along with general domain information.
http://www.topcoms.com/