For political junkies, this website has a very useful map showing who's leading in the latest polls in all of the states in the presidential and Senate races. The map is helpfully color-coded by strength of the candidate's lead. When you drag your mouse over a given state, it tells you when the last poll in that state was conducted, shows what the results were, and gives that state's presidential vote for the last four elections. The site is updated daily.
Right now, the site has Obama ahead in the electoral college, 287-227. It also has Democrats leading in enough states to produce a 58-42 Senate majority.
Note: the same site had John Kerry ahead in the electoral college through most of 2004, including in the days immediately before the election. The major problem was that the polls had Kerry winning Florida.
The site is only as good as the polls it relies upon, but it gives you a rough idea of where things stand at a given moment. I confess it's an almost daily stop.
UPDATE: Several commenters suggest an alternative site for looking at the state of the presidential race, www.fivethirtyeight.com, which uses weighted averages of polls taking into account things like pollster reliability and the age of the poll. While the site is interesting and contains much information of its own, I find it considerably less user-friendly. It also doesn't look at the Senate and House races. Finally, I'm leery of weighted averages since they raise difficult methodological problems. I prefer the electoral-vote.com approach, but no single site is perfect. Political junkies can judge for themselves which ones it's worth spending their time monitoring.