I don't have any contributions I'm really that proud of (enough to list here at least), but my favorite contribution from someone else (here, it's several people) has got to be:

Build a working game of Tetris in Conway's Game of Life (specifically the answers)

For reference, Game of Life is a simulation on a grid of living and dead cells. Each cell has eight neighbors, since corners count. If a living cell has 2 or 3 cells next to it, it stays alive; otherwise, it dies from loneliness or overcrowding. If a dead cell has exactly 3 living cells next to it, it comes back to life. This is actually able to compute anything computable, but is extremely difficult to manipulate. Yet, people managed to implement a real game of Tetris using such a simple system.

The question was posted in mid 2013. The answers were posted in fall of 2017. A team of people worked on this project throughout pretty much that entire period of time. The reason I like this so much is that in many ways, I think it reflects the nature of our site (Code Golf) - we're that one* weird site that doesn't do Q&A and instead has fun and does challenges for fun, yet even though it's oriented around competition, we have a really strong community, people love collaborating and helping each other on their competitive submissions all the time, and we're willing to put forth high effort for totally impractical things.

^{*actually we're not the only weird site; we're not even the only non-Q&A site, but I think we cause the most headaches in terms of our structure and needs (not in terms of causing trouble or whatever)}