Auction Patrol http://www.auctionpatrol.com An online auction portal. Has moderated chats, listings for resources, free image hosting, and lot more. Can also send auction-themed postcards. One of the coolest features is their auction calculator where you can compare how much it will cost to auction an item (currently only compares 3 auctions). They also have an acronym dictionary which is very handy for auction newbies and veterans alike.
Auction Watch http://www.auctionwatch.com/ An auction portal with searchable auctions, an auction manager and more. The discussion boards seem to be the most heavily used boards for online auction dealers (with possible exception of Ebay's own boards.)
The site also offers free picture hosting and some original content. Great tips for auction buyers and sellers.
BidFind http://www.bidfind.com/index.html Searchable index of current auctions from about 300 different auction sites (including ebay). They claim they were the first auction search engine on the Internet and are the largest auction data aggregator.
icollector http://www.icollector.com/ Auction catalogs and archives, but archives don't have completed prices; only estimated prices. Has a dealer directory. They do have an art price guide (no pictures). It contains more than 1,100,000 auction records of sales from auction houses in 40 countries from 1987 to 1998.