As long as it's "polling day" at the blog, this item from Nate Silver when he was still part of NYT is worth revisiting--it compares polls the three weeks before the 2012 election with the actual results. An excerpt (with some bolded emphases added):
Among the more prolific polling firms, the most accurate by this measure was TIPP, which conducted a national tracking poll for Investors’ Business Daily. Relative to other national polls, their results seemed to be Democratic-leaning at the time they were published. However, it turned out that most polling firms underestimated Mr. Obama’s performance, so those that had what had seemed to be Democratic-leaning results were often closest to the final outcome.
Conversely, polls that were Republican-leaning relative to the consensus did especially poorly.
Among telephone-based polling firms that conducted a significant number of state-by-state surveys, the best results came from CNN, Mellman and Grove Insight. The latter two conducted most of their polls on behalf of liberal-leaning organizations. However, as I mentioned, since the polling consensus underestimated Mr. Obama’s performance somewhat, the polls that seemed to be Democratic-leaning often came closest to the mark.
Several polling firms got notably poor results, on the other hand. For the second consecutive election — the same was true in 2010 — Rasmussen Reports polls had a statistical bias toward Republicans, overestimating Mr. Romney’s performance by about four percentage points, on average. Polls by American Research Group and Mason-Dixon also largely missed the mark. Mason-Dixon might be given a pass since it has a decent track record over the longer term, while American Research Group has long been unreliable....
It was one of the best-known polling firms, however, that had among the worst results. In late October, Gallup consistently showed Mr. Romney ahead by about six percentage points among likely voters, far different from the average of other surveys. Gallup’s final poll of the election, which had Mr. Romney up by one point, was slightly better, but still identified the wrong winner in the election. Gallup has now had three poor elections in a row. In 2008, their polls overestimated Mr. Obama’s performance, while in 2010, they overestimated how well Republicans would do in the race for the United States House.
Instead, some of the most accurate firms were those that conducted their polls online.
The final poll conducted by Google Consumer Surveys had Mr. Obama ahead in the national popular vote by 2.3 percentage points – very close to his actual margin, which was 2.6 percentage points based on ballots counted through Saturday morning.
Ipsos, which conducted online polls for Reuters, came close to the actual results in most places that it surveyed, as did the Canadian online polling firm Angus Reid. Another online polling firm, YouGov, got reasonably good results.