GlassBooth is a Web app that will quiz you on your values, and then select the perfect presidential candidate for you based on his or her public positions on the issues.
It's not the only site that compares issues across candidates (see also CNET co-founder Shelby Bonnie's PoliticalBase, and Minnesota Public Radio's Select a Candidate), but I find it the best of the bunch, for a few reasons.
First, candidate finder quizzes like this need to measure two things from users: what their issues are, and how important each issue is to them. GlassBooth splits this process into two clean parts. In the first, you assign points to the issues that matter to you. In the second, you define your positions. Other quizzes put the issues and importance questions on top of each other, which can make them confusing and tedious.
Second, GlassBooth appears to be backed by good research. It shows you the data it has on each of the candidates and their issues, and will link you directly to the source should you want to read more about the stance in question. Ultimately, GlassBooth gives you a numerical score for each candidate, ranked by best match to your ideals.
You can also explore the GlassBooth research on candidates without going through the quiz. I found the interface for doing so very clear, and the data in the system well-chosen.
My big reservation about most candidate finder quizzes is that the scores tell you very little about how well your candidate will govern. GlassBooth excerpts candidates' voting and legislative records, which provides some insight into this. I strongly encourage users to look beyond the cute numerical scores to find out how the rankings are derived. It still won't tell you enough about how good your candidate will be at working legislation through congress, but it will show if he or she does more than just talk fancy.
This is a great service.