Seeing as how we have quite a few Mount & Blade players here, I think it's time we collect our favorite mods. A couple requirements before you post a mod:
1) You have to have actually played it.
2) List whether it's a "native" mod (meaning it adjusts the default game) or a custom mod (meaning you load a non-native version of the game).
3) Link to the mod in question.
You can find a lot of general mod discussion on the official forums.
Here is a compilation of little tweaks and changes you can adjust in the game files.