sex.com sold for $11 million in January 2006
business.com sold for $7.5 million in 1999
auction.com just sold this year for $1.7 million in March 2009
poker.com sold for somewhere between $10 million and $20 million in a silent auction
FYI. Internet lore says that the domaining business started in the 1990s when someone who owned IBM.com sold it to IBM for $100,000. Before that, no one realized that there was hidden gold in domain name sales.
I once worked on a project with a company that owned IT.com. It has since been sold. I wonder how much that one went for. A lot of domains sell for high prices or are traded among domainers and we never hear about the prices.
Insure.com was recently sold for $16 million, so this might top all the other sales. However it's unclear if this is purely a domain sale or included some more assets.
Insure.com Sells For $16 Million