Which Track to Track
A big part of the vast haze of polling data out there this year has been the proliferation of national "tracking" polls, which keep up on a continuous basis with a reasonably stable sample of voters. They're popular for the obvious reasons that (1) they're available to feed the political beast every day, and (2) they're useful for following trends, even if the actual findings are suspect.
In case you've been ignoring polls up until now, it's time to wake up and smell the numbers. Fortunately, Nate Silver has now published detailed assessments of the eight national tracking polls, with all sorts of notes on their strengths, weaknesses, biases, and usefulness.
His bottom line is that Rasmussen's tracking poll seems to be the most reliable, though it's worth noting that two others--Washington Post/ABC and Gallup--are the only ones that include cellphone-only samples, which arguably gets at an important source of votes for Barack Obama. Interestingly enough, those two tracking polls (for Gallup, the revised likely voter model that doesn't weight results according to 2004 turnout patterns) have Obama up by nine percentage points, while Rasmussen has the margin much lower, at four percent.