Rick Santorum "suspended" his campaign today, but he's really out. Newt Gingrich's campaign is beyond broke. Ron Paul is still campaigning, but that's fairly irrelevant, at least as far as who will be the Repub nominee. It will be Mitt Romney.
The NYTimes coverage includes video of Santorum's eloquent, dignified withdrawal statement. He didn't mention Romney and hasn't endorsed, but the two talked and made nice.
Santorum lasted longer than many other Repub candidates who were rated more serious candidates by the handicappers at various points, including the two Minnesotans, Tim Pawlenty, who didn't even make it to the Iowa caucuses, and Michele Bachmann, who made it till the morning after Iowa.
Santorum got further with less money than is supposed to be possible. There may be some talk of him as a Romney running mate, but that seems unlikely. If Romney loses this year, it's easier to believe that Santorum will try again in 2016.