The Political Arithmetik blog has a steady stream of fascinating and well-presented graphs having to do with various political polls.
That blog's national support for 2008 Presidential candidates graph has unfortunately not been updated since August 7, but I'm guessing it'll be updated again soon. (Oh--see below.)
In the meantime, there are all sorts of interesting graphs showing Presidential approval ratings, elements of public opinion about the Iraq war, the effect of multiple polls from the same company on state primary/caucus poll trends, and so on.
One thing I like about the blog is that it's not just pretty graphs; it's also smart and interesting analysis of the data and the trends, by Prof. Charles Franklin, a poli sci professor at the University of Wisconsin.
And it's also pretty graphs; most of the other poll-aggregating sites I've seen don't have graphs, and most of the ones that do aren't very attractive.
Prof. Franklin is also associated with Pollster.com, which has a more up-to-date Presidential primary page, including the pretty graphs (but less analysis); the latest poll date there is 12 September. Each graph on that page is linked to a much more detailed page showing all of the poll data that they're using. So I think this Pollster.com page is probably the main one for me to bookmark.
Partly I'm linking to these because every time I go looking for graphs of poll data over time, I find a bunch of sites that don't have graphs, and have to dig down into the search results to find these ones, so posting this entry will make it easier for me to find the sites in the future (and may give them a bit of a boost in the results listings too, though I suspect there are already a whole lot of higher-profile links to them).
But mostly I'm linking to them 'cause I thought some of y'all might find it interesting. For a bunch of quick snapshots, look at Political Arithmetik's right-hand sidebar, and click any graphs that look interesting.