One word:
TorchlightI've made a topic about this game before, but I'll just rewrite some stuff.
Torchlight is a dungeon crawling single-player RPG (although an MMO version is in production and a multiplayer mod is being worked on) that brings pretty 3D graphics to a very Diablo-esque game. Developed by people who worked on games like Diablo 1 & 2, Fate, Mythos, and others.
It has a lot to offer and is just a great game in general.
But the important part is, they actually released
the program they used to create the game. If you know how to work it, just about anything can be done. Create custom classes, items, NPCs, quests, maps... everything. Even without the tools though, it's possible to create/edit the files needed in the correct folder. Now, there's all sorts of mods already made and the game was only released a few months ago.
I'm currently modding it myself in my spare time. Nothing done yet, just screwing around for now. I have plans to get something released though.
Anyway, that's about all I gotta say for now. Maybe, if I finish anything, I'll post the results here too.