Digital PC/Mac game distributor Direct 2 Drive isn't currently having a pre-holiday sale to compete directly with Steam's weekend-long sale, although they do guarantee matching the lowest price on games from certain online retailers, including Steampowered.com. However, it is advisable to closely read their agreement before attempting to take them up on this offer, as it appears to exclude sales, promotions, coupons, and bundles, and is only applicable for a limited time on games that have already been purchased. The "price matching" is done by giving the purchaser a store credit, so it is not the same as directly matching one of Steam's sales.
So, although there aren't any special sales just for this weekend, D2D is having a couple of giveaways that last now through December 31st, 2010, explained on their D2D Gives Back 2010 page. First, they're having a tweet and win give away. Follow the link to tweet a prewritten message with a #D2DGivesBack hashtag, and you'll be entered for a daily drawing of $50 in store credit. This is simple enough and comes at no cost to the entrant.
D2D is also having a "1-in-5 orders is free" promotion. To participate in this, you must order something with a credit card. After your purchase has been completed, you will be notified of whether or not your order was chosen to be free. This is the part of their giveaway that, while it is good for their regular customers who were already planning to buy, strikes one as being a gimmick to drum up sales. They are giving back, but they are only giving back to customers. Most giveaways do not require a purchase up front. In fact, I would not call this a giveaway at all; it is more like purchasing a chance to win back the price of your purchase, and one may only win once per day. Although it is better than nothing, it's not a giveaway, and it's not a very generous promotion. It may encourage many people to buy something simply so that they can gamble on the chance of winning, and some will come away disappointed.
If you were already planning to purchase from D2D, now might be a good time to do it (if the game is not on sale elsewhere), as your purchase includes what is effectively a raffle ticket for a 1-in-5 chance to not be charged for your order. Certainly, it costs you nothing to participate in their Twitter giveaway, so the promotion page is worth taking a look at for that.