This may be a bit out of line as it is not a movie, but I can truly recommand a great post-apocalyptic TV-series called Jericho. It's currently running here in Norway now, and episode 16 will be aired today. If you are interested in knowing a bit about it, you can find the official homepage via Google by searching for Jericho. In short, it is about a small American town and their people after the US has been nuked.

As for a movie, well, I could probably recommend some old ones, but there are none of the new ones that comes in mind.