For the Democrats Al Gore is currently the only also-mention for president. I don't think he will run, but if he does we will have to hear from idiots who voted for Bush yet have the audacity to pick on Gore for his "I invented the internet" comment which, while a dumb thing to say, was linked to something in reality (though it is a reality which other US Reps and Senators can claim). I like Gore lots more now than when he was VP or running for President. I think he can do more (not inter-) net good with less stress by not running (while it is possible he would be the best president, there is no one on the ballot who can replace him in his current endeavors). Wes Clark is also mentioned a bunch on the net, but I think he had his place in the last election, and there are too many superior candidates to him already running in this one. Note for him though: he is a very strong supporter of the sciences, and is both knowledgeable and curious about scientific issues. The other candidates are mostly a collection of "whos" and "whys": Baya, Biden, Dodd, Kerry, Kucinich, and Vilsack, though Vilsack will do well in his home state of Iowa.
On the Republican side McCain and Giuliani are the apparent front runners. I think the former will be dead thanks to Bush's support for his troop surge (which will fail). The latter is probably the most likely nomination, but he's still a bit of a dark horse. He's from New York, which makes him not a favorite son of the party, and there is a perception that he is more liberal than others which is probably true. Of course he is also a huge supporter of big buisness and heavy handed crime prevention, so he'll probably manage it. Brownback is a likely GOP spoiler. He is the type of candidate to do well in a GOP primary and tank in the national. If he wins, pretty much any Dem short of Kucinich would coast to the White House...probably. He also has a fair chance of winning, especially if McCain's support dries up before the primary. Dem spoiler potentials include Giuliani, but also Romney and Pataki. Probably the only Republican who would be a favorite in Nov '08 would be Colin Powel, and there is no chance he will run after the shit storm surrounding this administration that he was part of. Giuliani and currently McCain are probably even at best, and McCain certainly won't stay there.