I love these sorts of novels too. Here are a few suggestions:
Well, obviously, there's King's THE STAND (which I'm re-reading right now, as a matter of fact -- God, I'd forgotten how great this book is) . . . .
Also just bought DARK ADVENT by Brian Hodge. Haven't read it yet, but it's supposed to be a great tale of a group of survivors at the end-of-the-world.
THE RISING by Brian Keene is a great one. Definitely check that out, if you're a fan of either apocalyptic fiction and/or zombie fiction.
Heard good things about FIRE, by Alan Rodgers. And LUCIFER'S HAMMER, by Larry Niven (the latter's more sci-fi than horror, though). I'll probably give those a whirl soon.
Hope this helps,
James
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