The Steam Holiday Auction lets you trade in Community items like trading cards, backgrounds and emoticons for honest to goodness games.
UPDATE 2: The Steam Holiday Auction will be back shortly now that Valve has fixed the issue with gems and had to "reset and start again."
The good news is that all gems have been returned to user inventories, meaning they are not lost forever. All gems purchased from the Community Market will also be reimbursed, and all auction bids have been reset to zero.
Trades involving gems are "being investigated on a case-by-case basis."
Still no ETA on its return other than "shortly."
UPDATE: Valve has temporarily closed the auction due to an exploit.
"Sorry, but there have been some issues with Gems and the Steam Holiday Auction has been temporarily closed," reads the statement. "The elves are working frantically to get the issues sorted out, and the auction will start again as soon as they're done."
Original Story: Steam has announced an unusual promotion allowing you to recycle to contents of your Steam Community inventory into gems.
These gems can then be used to bid on games. Over 200,000 games are on offer, and a new auction starts every 45 minutes.
This is potentially pretty lucrative; over 1,200 games hand out trading cards just for playing them, and Steam members can craft these cards into badges, which have a much higher gem value than the cards on their own.
Gems (and the items needed to make them) can be bought or traded for on the Community Market, so even if you don't want to bid, you can potentially get something cool out of all your free Community guff.
The promotion starts Monday, will run through 10:00AM PT on December 18, and sounds suspiciously like a an experiment by Valve's in-house economist. Visit the promotion page for more details.