I’m involved in several open source projects.
One day I’ve got completely pissed off maintaining a large Maven/Tycho project. So, I’ve written a Maven plugin to protect myself from people, who don’t know what they’re doing. It helps to ensure build integrity of a multi-module project with branchy structure.
PEG parser in Fantom. Special feature of this parser is that it allows to parse really big files, even if the file and/or the parsed tree wouldn’t fit into RAM. Also, it is incremental: you can parse a part of a text, then stop it and parse the rest of the text afterwards. Finally, meta grammar is not hardcoded and can be changed using the parser’s API (only PEG expressions are hardcoded).
Fantom library to find differences between two sequences of objects of any type (e.g. strings, text files line-by-line, etc).