OnTheIssues.org is a nonpartisan website dedicated to providing information and sources on different candidates' positions on a wide range of issues. The website provides quotes from the candidates on topics such as abortion, civil rights, corporations and education, and links many of these quotes to their original sources for verification.
The official websites of those running for office usually provide their positions on selected major issues. Televised debates and summits are also good sources that allow the voter to see the sometimes subtle contrast between the stances of seemingly similar candidates. C-span airs and archives some of these debates and summits.