NY Times has a chart on the candidates, and I've added it to my main post on this subject.
Obama has come out with a strong plan after flirting with the coal vote for a while. Edwards has tightened up his cap-and-trade from saying some permits should be auctioned to saying that all permits should be sold following a transition period.
Obama's call to match 1990 levels by 2020 is comparable to Edward's cap at 2010 and 15% reduction from the cap by 2020 (Obama's might be a little better, but Edwards has made this a priority). I don't see an equivalent short-term goal at Clinton's website, though.
On the Republican side, Thompson has allegedly started equivocating on his denialism (I'll resist the urge to take any credit for this). I'd prefer some actual quotes from Thompson though. My guess is that he's equivalent to Bush 2005-2006, saying different things according to what the audience expects. Giuliani is only a smidge better, saying there's a human contribution to warming but not how much. Romney is equivalent to Bush today, acknowledging the problem but not wanting to do anything about it. McCain is a significant improvement, although not as good as any of the Democrats.
UPDATE: He's not a major candidate, but according to the Times chart, Ron Paul is a disaster on climate change.