Modding is an essential part for any long lived game, those games developed with modding in mind usually tend to sail for a longer time, while other titles die out more quickly. FeralHeart is a good example too, if we weren't able to make GUI's, texture mods, marking packs, sound mods and what not, I'm very confident that we wouldn't have half of the userbase we have today.
I do enjoy seeing devs that are very open minded towards modders, something Valve does very good. If you guys know Portal, as far as I heard it started out like a fun Half Life2 mod. Garrys Mod is another example. There's many more titles that started out just that way.
Personally I like graphic enhancing mods and Co-op mods for games if they exist.