Cryptonomicon

A fabulous epic. Stephenson combines his erudition with a delightful sense of (geek) humor to bring all the characters to life. Particularly fascinating is the infusion of real life characters (Alan Turing), interacting with ficticious characters. The convergence of parallel (3?) storylines toward the end was handled superbly.

Look out for Bobby Shaftoe, the marine raider, easily the most entertaining character.

Not your average sci-fi, but it still has pretty pretty interesting concepts (like "van eck phreaking").