Hitchhikers Guide To The Galaxy by Douglas Adams. The super computer Deep Thought took several hundred millinia to calculate the answer to 'what is the meaning of life' and finally came up with the answer. 42. If you havent read it you really should. Its HILARIOUS!!!!
Like the others mentioned, it's from Douglas Adams' hilarious "Hitchhiker's Guide to the Galaxy" series. The meaning of life was given by the supercomputer Deep Thought as 42, and when the race that built it complained that this made no sense, the computer replied that neither did the question, and that they neede to build a different computer program to find the "ultimate question".