Dave Leip's Atlas of U.S. Presidential Elections. This site presents presents state-by-state voting results for elections from 1789 to the present. The focus is on the presidential elections (as suggested by the title) but there is also information on governor and senatorial races. There is also information on primary results.
This site has a lot more too. There are predictions for the 2006 midterm election, a wiki, several forums, and tons of data files. Some of the files require a paid membership to access. All in all, this is a fun and information rich site.
From the site:
The Atlas is a free internet resource providing results of U.S. Presidential Elections to the world community. Data is collected from many official sources and presented here in one convenient location. Site membership is now available for those individuals or groups who desire greater levels of detail with regard to U.S. Presidential Elections.