It looks like Sam Brownback, the social conservative once closest to my heart, is about to depart the presidential race. At first glance, this would appear to benefit Mike Huckabee, who was competing for a lot of the same donors and many of the same voters in Iowa. The former may still be the case, but recent poll numbers suggest that, in terms of on-the-ground support, Brownback had ceased to be much of a threat to Huckabee.
One somewhat longer shot is that Brownback ends up getting behind Mitt Romney. I realize this sounds pretty counter-intuitive--Brownback and Romney famously went at it this summer over abortion, with Brownback essentially calling Romney a fraud on the issue. But endorsing Romney has the potential to transform Brownback into a king-maker. And there's also this: Brownback's campaign manager and chief strategist, Rob Wasinger, is actually pretty friendly with Romney's deputy campaign manager and unofficial social-conservative liaison, Peter Flaherty. (Both men, like Brownback, are devout Roman Catholics.) It wouldn't shock me if they've been having conversations about this, or even if that's one reason Brownback's dropping out at this point.
Update: Ambinder thinks it could be either Giuliani or McCain. Giuliani I don't really see, since he doesn't appear to be competing in Iowa (see below), and since he's explicitly pro-choice. But McCain strikes me as a possibility. As the AP reports, he and Brownback are pretty friendly. And McCain is probably closer to Brownback on abortion than anyone he could plausibly endorse, save Huckabee. (Thanks also to commenter jhunger80 for the heads up about McCain.)
Update II: Mike Allen and Jonathan Martin say a Romney endorsement is unlikely. They quote Brownback media consultant Paul Wilson to this effect, though it's unclear if he has information from Wasinger, or if he's just basing that on what he's surmised from this summer's squabbling.