Filter for converting upper case HTML tags to lower case. It helps a lot when converting old HTML files to XHTML that doesn't allow upper case tags. It also preserves case of inlined programs (e.g. PHP) and quoted strings.
Set of functions for easy building of a call tree of functions of a given program. It could greatly help in understanding how does an unknown piece of code work.
My very old school project - NetShips battle, written in C/C++ running in Linux, of course. I'm still taking care of it - at least it still compiles even with the new GCC 3.x...