Runoffs in North and South Carolina, and the post-convention primary in Utah. Three big races: SC-Gov (R), NC-Sen (D), and UT-Sen (R). Also, we'll presumably lose a Member of the House tonight, with Bob Inglis presumably getting knocked off in a runoff. The invaluable Taegan Goddard, as always, supplies poll closing times.
I'll send you to Ed Kilgore's nice analysis for a rundown of what's happening, and I'll just make a couple of comments going in. I still don't understand why national Democrats are so sour on Elaine Marshall, who is locked in a tight battle with Cal Cunningham for the chance to take on Richard Burr; Burr isn't polling very well, and is probably the weakest GOP incumbent Senator on the ballot this November. In South Carolina, Palin-backed Nikki Haley will probably win. For what it's worth, I can give a bit of my own observations here: I watched a chunk of the runoff debate, and she comes off much more as a mainstream conservative candidate than as a Tea Party flake (of course, that's based on maybe half an hour of one debate, so I wouldn't rely on it over actual reporting).
As always, I urge all to vote early, vote often, and celebrate democracy.