US National Archives
Nara: Presidential Election Laws: Presidential Election Laws
Check out this site from the National Archives and Records Administration on the US Constitution and the United States Code for laws and provisions regarding presidential elections.
This Nation
This nation.com: Electoral College
A textbook excerpt from ThisNation.com that includes a useful chart of electoral votes by state. Clickable links to pertinent articles and amendments of the Constitution are another plus.
Other
Post Gazette: 23 District of Columbia Suffrage
This resource contains a brief explanation of what the 23rd amendment does and where it originated.
Other
Democracy and Diversity Voting Rights Quiz [Pdf]
How much do you know about who can vote in the United States? Which amendments to the Constitution govern voting rights? Take this quiz to find out.