There have been a lot of good movies listed in this thread, but I can't believe nobody has mentioned (unless I missed it) Glen and Randa, which is about as classic and thoughtful a post-apocalytpic tale as has ever been put onto film. It's a story of two young "tribals" who set out, very unprepared, on a journey across a very well-designed wasteland world populated by colorful characters. The "mood" of the movie is something that any Fallout fan would appreciate.
I second The Survivalist and Idaho Transfer mentioned on this page; both are low-budget but interesting and well done "low profile" movies. Steve Railsback (The Survivalist) was also in another decent post-apocalyptic flick called Escape 2000. Despite the goofy title, it's worth watching.
I sort of enjoyed an on-theme B-movie comedy from the 80's called Radioactive Dreams, about two guys who grow up by themselves in a "vault" and recieve their entire education about the outside world and the past from some 1940's detective novels left sitting around. They then cause a big culture clash as they set out across the wasteland in search of their long-lost father. It's as strange as it sounds, but it had its moments.
Dean Cain had two respectably good post-apocalyptic movies a few years back. One, called Final Encounter, actually plays out almost like a game. Two dwindling factions from an ancient war, who continue to despise each other despite not actually seeing battle in decades, search for a way to blow up their world once and for all. The other one, Post Impact (also reminiscent of a game plotline), is about a group of soldiers and scientists who have to travel into a frozen wasteland left over after an asteroid hits the earth in an effort to salvage some old technology. They're both first-rate second-rate movies, and worth a watch.
