The ultimate yakshave
If there is one software package just named for immature jokes, it's Leslie Lamport's TeX. It has an incredibly long history - and that on top of another software package with a long history: Donald Knuth's TeX.
Both were created out of necessity and dominate a pretty arcane field: typesetting. I love the history of TeX, as it is an interesting story of tradeoffs, non-standard requirements and solutions that took longer then expected.
If you ever think "we should replace TeX, should be a quick thing", I'm here to convince you otherwise.