OnTheIssues.org is a pretty comprehensive "just the facts, ma'am" resource for the Presidential candidates as well as state races. You'll basically get info from their voting records and some public statements they make. For one-stop shopping, that's as good as I've found. The devil is always in the details, but for simple high-level views of where the candidates stand, you can start with the candidate grid by topics or the frontrunner grid, and then drill down to the specific votes or statements which led to that assessment.

There are also a bunch of "answer a bunch of questions and we'll pick your ideal candidate(s)" sites, but I think they're bogus, and in some cases, bankrolled by certain political interests.

If you want to wade deeper into the muck, political blogs can round out your knowledge, but the signal-to-noise ratio can get a bit low at times.