Hello everyone, I'm wondering if there are some mods that can change the UI, improve the game, fixes bugs ecc...
I'm not searching for single player campaigns mods, as the topic title say.
Are there any out there?

Yes, several. They go into your override folder. For example:

GUI changes (My personal favorite is this, but I think very popular are e.g. all the mods by The Amethyst Dragon)

I also like Kamiryn's Customize Character Override (I hope it's the one that I remember), which gives you more options to change the look of your characters and their equipment, even after creation (you can e.g. dye your clothing, those default tan brown armor colors are rather ugly).

Then there's Tony K's Henchman Inventory & Battle AI Mod which slightly improves the handling of your henchmen and their battle tactics (and I think also the AI of monsters and NPCs).

If you don't like the textures and tilesets, you could try NWNCQ which replaces them with new graphics; be aware that it's somewhat experimental though, a (abandoned?) work in progress, at least the last time I tried it, it wasn't flawless, but it's still a pretty cool project. I used it during parts of the original campaign, when I got bored by the bland look of the default tilesets. Not for purists!

There's also Project Q which is similar to the above, and additionally replaces monster models with mostly improved ones (as always, it's a matter of taste, of course). Not sure how it works exactly, whether this can be put into the override folder as well; you'd have to read the installation instructions.

Well, these are just some examples off the top off my head. I'd have been quicker to provide help and links when the old NW Vault was still up and the old Bioware forums were still online and I was more involved with the community. Nowadays it's a bit harder to get a good overview, many link compilation have been lost or are harder to find. There's also something called PRC that many people like and that I believe adds additional character classes and races, but I don't really have a clue about it. Hopefully someone else can step in and add to the above.

Also note that naturally you can't have everything at the same time, not all mods are compatible.

Thank you very much.
:)

Try here:
http://lilura1.blogspot.com/p/nwn-utility-visual-mods.html

Also for NWN2:
http://lilura1.blogspot.com/p/neverwinter-nights-2-mods.html

Pretty cool, thanks! There are several mods among them that I haven't heard of before, I'll have to try The Barbarian heads etc. next time I play NWN. Do you also know about any tool to manage your override folder? There was one back in the days but I forgot its name and I don't know whether it's still available and working. You could create several different override folders in your NWN directory and it would allow you to choose between them before starting NWN (copying the specific folder's content into the official override folder).

This one apparently lets you configure overrides but I haven't tried it out myself.

https://neverwintervault.org/project/nwn1/other/tool/neverwinter-nights-mod-installer-tool-nit

It's a fairly recent release, I think, so it's probably not the one you had in mind. Still, the author is pretty active so maybe PM or post a request on that entry if his tool doesn't suit your needs.

That looks pretty cool. Bookmarked, thanks!