Work related project was a library for curves representation (polynomial, bezier, and a lot of other types) in C++. I liked working on it for several reasons. First one is that I could finally start something from scratch after years of working on legacy code. No dependency on strange old library from the team, only mainstream libraries.

But mostly it was because I learned a lot on this project. I had to mix template programming, heavy use of polymorphism, python bindings of the c++ and serialization together. I had experience in all of this stuff already, but mixing everything together bring a lot of new troubles and you have to understand how it works more in deep to be able to solve them.

I'm not making "famous" open source package with thousands of download and used everywhere, but seeing this package still in use in several other projects (and not only in my initial team) even after I left the initial team feels good. One day someone from my new company recommended to use "my" library as dependency to solve one of our problem, without knowing that I was the author, saying that it was a good well written lib. That's a nice ego boost!