This link may have been discussed here before, but it's still pretty cool.

Abstract
This report describes my successful project to build a working reproduction of the 1964 prototype for the Block I Apollo Guidance Computer. The AGC is the flight computer for the Apollo moon landings, and is the words's first integrated circuit computer.

I built it in my basement. It took me 4 years.

If you like, you can build one too. It will take you less time, and yours will be better than mine.

I documented my project in 9 separete .pdf files:

Part 1 - Overview [8.1 MB]: Introduces the project.
Part 2 - CTL Module [9.9 MB]: Design and construction of the control module.
Part 3 - PROC Module [6.7 MB]: Design and construction of the processing (CPU) module.
Part 4 - MEM Module [6.8 MB]: Design and construction of the memory module.
Part 5 - IO Module [7.0 MB]: Design and construction of the diskplay/keyboard (DSKY) module.
Part 6 - Assembler [0.5 MB]: A cross-assembler for AGC software development.
Part 7 - C++ Simulator [5.2 MB]: A low-level simulator that runs assembled AGC code.
Part 8 - Flight Software [2.8 MB]: My translation of portions of the COLOSSUS 249 flight software.
Part 9 - Test & Checkout [0.9 MB]: A suite of test programs in AGC assembly language.

Why build an AGC?
Early computers are interesting. Beacuse they're simple, you can (if you like) actually understand the entire computer, from hardware to software.

The AGC is the most interesting early computer because: a) it flew the first men to the moon; and b) it's the world's first integrated circuit (IC, or microchip) computer. It also has interesting architectural features.