A few weeks ago I finished Cryptonomicon (more info here), by Neal Stephenson.
I really enjoyed it. Without giving too much away, it deals with cryptography, World War 2, Alan Turing, math, society, global finance, intrigue...I could go on, but I won't. If you have any interest in any of these topics, or none of these but you like a really good book, pick it up!
I've also read Stephenson's Snow Crash in college, and it was excellent as well.
3/4 update - we were in Goodwill last night and I picked up Snow Crash for $2.99!