NPR did a survey to find the top 100 Sci-fi and Fantasy books. Below is the list, and I've checked off the ones I've read.
Oddly, some are listed as a series, and some are individual books. For instance, The Caves Of Steel is listed instead of The Robot Trilogy and notably Going Postal instead of The Discworld Series (Going Postal is book 29 of the series).
Update Sept.5 2012: Since I posted this a year ago, I've read (or listened to the audiobook of): The Kingkiller Chronicles, A Canticle For Leibowitz, The Forever War, Wicked, and A Fire Upon The Deep, so I checked those off. Might just get through this whole list.