One of my favorite themes. Here are some titles I've read recently:
The Rift by Walter Jon Williams
A huge earthquake in the Missippi Valley region. A GREAT read. Believable plot, great characters, conflict between heroism and evil, a sprawling tale. Here, the entire earth is not affected, only a huge portion of the US South. It is more realistic than the global disaster plots - in fact, when the current devestation in the Gulf area happened, I thought of this book.
The New Madrid Run by Michael Reisig
A huge tidal wave is the disaster, and the book follows the story of some survivors on a boat, later on land. Good read, but I have some reservations about the writing, it is more on a teen-age level - but still worthwhile.
Earth Abides by George R. Stewart
My very favorite of this type. Wonderful. I've recommeded it many times. Far more intellectual, less sensational, more thought-provoking. Read it. The main character survives the disaster (a worldwide virus) due to a freak accident. The book covers the rest of his life with the few survivors. It has much to say about society, how little would be preserved in such an event, how quickly the land would return to a wild state.
No Blade of Grass by John Christopher
A virus kills of ALL strains of grass - including not only turf but rice and barley, causing world-wide famine and panic. It takes place globally, but the plot focuses on a band of travelers in England. Excellent read - I highly recommend it - but it may be hard to find.