Piling on a bit here, but yes, bar mitts are the thing. I have a pair of Bar Mitts for my MTB (flat bars) and cross bike (drop bars), both which see commuting duty. Nothing else will keep my hands warm below 15 or so (well, short of not riding). The drop bar version seems a bit tight for me, and makes it a bit tricky to get the best angle on the levers when braking, but I think that just means I need to upgrade to discs...