UPDATE: The now-best chart I've seen so far is at the NY Times.
Nice resource on the main candidates' positions put together by the Council on Foreign Relations, of all the groups to have first thought of doing it. It's not bad, although they should've linked directly to the candidate's sites also for updates, rather than specific articles scattered around the internet. Someone should take it to the next step with a more comprehensive evaluation.
Looking around some more...
Okay, an article in the Christian Science Monitor. More anecdotal than comprehensive though. The statement, "All the major candidates say global warming is real, that it's caused to some extent by human activity" won't hold true if the dumber-and-lazier version of George Bush, Fred Thompson, enters the race. UPDATE: Thompson may be backing off a little.
This is the best I've found so far, a comparison chart from the League of Conservation Voters website. Some of it is hard to believe, frankly. Richardson wants 90% emission reductions by 2050? Besides the fact that it won't happen, I don't even think that's necessary. Chris Dodd says new coal plants must sequester emissions - that translates into "no new coal plants for a decade or two" which is fine I guess, but he might as well acknowledge it. Looking at the chart emphasizes the need to know what concrete, short-term steps each candidate would take as President, as opposed to lofty goals for the future. UPDATE: Chart incorrectly says Edwards would require sequestration from coal, when he would only require the capability to sequester.
More League info on the candidates and warming here, although the chart is the most useful part.
Brookings Institution set up a program to discuss ideas for the next president called "Opportunity 08" that discusses climate change. Earth Day Network blogged about an Opportunity 08 forum by the policy people for several candidates on climate change here (Brookings can't be bothered with reporting about it though, apparently).
That's the best stuff I found so far - I'd love to hear about other resources.
UPDATE: NPR has a chart, although not as good as the League/Heat is On chart above. I expect the two best climate positions are Edwards and Richardson (although Richardson is skimpy on details), so if you're interested in how other candidates stand up, these are the ones they should be compared to.