I recently bought an old (and out of print) game supplement for the 3rd edition D&D game. When the Sky Falls was published by Malhavoc Press in 2003 and has since become available both as a softcover book and as a download. At 64 pages its a slender volume but in its pages contain all you need to know to change your game world forever.
This is described as an event book and was produced under the Sword & Sorcery brand title in the boom following the adoption of Open Source rules for D&D. Here's a description from the Malhavoc site:
"This event book serves as a guide for DMs and players whose world is struck, and struck hard, by a meteor. Following the model of the popular event book Requiem for a God, D&D Epic Level Handbook designer Bruce Cordell brings you When the Sky Falls. This book presents a variety of new prestige classes for characters close to ground zero, a variant magic system powered by residual meteor fragments, and many new feats and spells. When the Sky Falls also describes frightening new monsters -- creatures twisted by their proximity to the Afterfall."
Although the rules were designed for 3rd edition the core of the book could be adapted easily. As a sourcebook for ideas I think it still works. There is a great review on the Geek Native site which is well worth reading.
Oh and the best bit, I picked this book up from a 'sale bin' for one pound. I do love a bargain!