I have made quite a few mods for RimWorld under the name Uuugggg. My mod Replace Stuff is the highest-rated RimWorld mod of all time (out of mods that actually add a significant chunk to the game), but I quite like them all. The TD Enhancement Pack adds countless little useful features to the UI. And honestly, Ctrl-F is my greatest achievement for its depth and extensibility, and it is quite under-used.

My mods mostly use C# code to add quality of life features and UI improvements. I play the game, run into an annoyance, and make a fix for it. I make no significant changes or additions to the actual content of the game – keeping the game Vanilla, just improving the experience. 

Get the game:

Project Highrise

While I have your attention, I gave a similar treatment to Project Highrise – With my mod, you can shift-click to place many units at once instead of tediously and precisely clicking hundreds of times, once for each unit (among various other improvements).

Endless Sky

I also contributed to Endless Sky: I added sorting of the mission list, other little features, and fixed some obscure bugs and quirks in C++. But this is an open-source game, so those changes are integrated into the release, so you don’t need to download a mod or anything. I just recommend the game.