For the republicans, Giuliani and McCain keep swapping first and second place about every other poll. After those two it's Newt Gingrich at a fairly distant third, but I think the more hard-righties might be able to make it happen for Gingrich if he decides to run.
In the Blue corner, Hillary's in a very strong lead over Obama, who has Edwards and Gore (Gore's said he's not into it, but there's still speculation and interest) trailing closely.
In the let's pretend General Election, it's hard to tell. If you sort of mentally average all the polls sited here, it looks like all matches between Clinton or Obama and Guiliani or McCain are within just a couple points either direction.
Already a nailbiter.


And now, a big pile of links for your 2008 election monitoring convenience:

Official and potential Democratic candidates

Official and potential Republican candidates

Official and potential third party candidates

Public opinon polls (watch this one)

Race speculation and candidates' issue stances

Senate Roll Call Vote Tables