The past several decades have seen the rise in popularity of the polling aggregator. These news websites build on the premise that more data is better than less data. They collect all the polls being publicly released for national and state races and provide a moving average of the horse race. [...] Survey aggregators provide a service in bringing the breadth of data together, but we recommend that readers not take the overall distillation into a single number as gospel. The best aggregators give you tools to engage with the data more closely, to track individual polls or different types of polls separately, and to understand how much of the movement is signal and how much might just be noise. -- Pew Research
FiveThirtyEight (statistical modeling)
RealClearPolitics (simple average)
The Upshot (statistical modeling)