Checkbox-Tagging
 

I don't want my users to fill in the tag-field with their own tags. Instead I want to let them select (checkbox, multiselect) x tags from a list of predefiened tags.

(Don't mind my english, I know it sucks ;))

There already is a mod for this - try a search.

All mods I can find are some kind of auto-tagging. Maybe I'm just too stupid to find them.

It isn't exacly like what you want, but you can modify it to do what you want, I would think. Or, try other search terms like "*define*" or "taglist".
Add Pre-defined taglist to showthread page

Ah, thanks a lot!