The US Election Atlas, located at http://uselectionatlas.org, is an online compendium of election results for the United States. It focuses on Presidential elections, but also has information on Senatorial and Gubernatorial races. It has apparently been a web presence since before the 2000 election. It is run by a "Dave Leip", although the site doesn't present any biographical information on him.
The site lets you pick a Presidential election from 1789 to the present, and presents a map of which candidate won which state, and by how much. For recent election cycles, it also gives county level data, either as a map or as numbers. Its information on gubernatorial and senatorial races is more limited, only going back to 1990, but is still a valuable resource.
It also keeps records of polling for both primary and general elections, although it presents this data uncritically, unlike (for example), fivethirtyeight, which weighs polls. It also includes a forum, which I am not as familiar with.
As a research tool for people studying American political or electoral history, the US Election Atlas is invaluable. Its interface is fairly straightforward, there is no flash or other web intricacies to slow it down, and as far as I can tell, there is no political agenda behind it. It is just a clear, easy source of information.
One final note: since the site predates the "Red State/Blue State" dichotomy popularized in 2004, it uses red as the color for the Democratic Party and blue as the color for the Republican Party. This may confuse new visitors at first!