As found on Vincent’s Twitter feed, SparkFun has an article on hardware hacking techniques:

It’s not a new article, but I’ve never seen it, and it’s informative, so I don’t care when it was written. 🙂

The Common Methods of Hardware Hacking. The article describes 4 methods of modifying hardware: Patching Into I/O, Replacing a Component, Logic Analyzer, and JTAG Hex Dump Voodoo

https://www.sparkfun.com/news/1314