Political quant Nate Silver performs a post-mortem on polling organizations, and finds a fairly widespread bias favoring Republicans (see here). Of nearly two dozen major polling groups, all but four were biased toward Republicans, which might explain why so many people believed that the presidential election would be closer than it turned out.
Among the more reliable (least biased) polling organizations were Mellman, IBD/Tipp, Grove Insight, Quinnipiac, We Ask America, and JZ Analytics. The worst by far was Gallup (among the oldest and previously most respected sources of public opinion polls), with a bias of 7.2 points in favor of Republicans, followed by the American Research Group, with a bias of 4.5 points in favor of the Republicans. The most bias favoring Democrats (2.5) came from the Pharos Group.
In future directions, RealClearPolitics and similar accumulators of opinion polls might consider using weighted averages based on Silver's bias rankings.