WC3 JASS.com
"The Jass Vault" plus vJASS and Zinc

Ok, I have seen some maps that have some kind of GUI that shows f. e. the hero, his items, where he holds them etc etc. and I do belive I would need (or atleast like) to have something similar to that in my own arsenal of things to do, and especially in the map I am currently working on.
 The thing is; I didn't even know it was possible to make that in wc3, even less how. If someone could explain it to me, I would be very grateful! (:

ZOMG!!!! working in a inventory system is a big task. The first thing to have into account is that there's no generic way to do this. Some RPG empathize some stats and others do the same with other stuff. Doing this UI implies a lot  of tricks. I've seen some inventory systems @hive and Wc3C but being a general thing implies that it's very likely that they don't fit in your map completely. Good RPGs make their own inventory system so they can fit perfectly. I'd suggest to check some of them (I don't have he links ATM) and check their codes.

(Item und Backpacksystem posted by Moyack is a good example)

Posted by me?? I don't remember

Darn I have a really awesome system for it that is pretty easy to read (in my opinion), but it is on my old computer, and it is not hooked up. :<

I'll try to get it for you later and show you a sample. Until then, you should probably look at Anachron's:
http://www.hiveworkshop.com/forums/spells-569/vjass-system-custominventory-159130/?prev=search%3DAnachron%26d%3Dlist%26r%3D20

Overall, most of the custom UI systems are coded to be very map specific. There is a way to make it more systematic, but you have to be a bit creative. It is somewhat demanding of the user overall. I tried to make an elegant approach that does a lot of the work for you. For example, that is why I wrote systems like:
https://wc3modding.info/5016/system-wordwrap/

You'll probably encounter the problem that the system relates to as you get further into making your system. Just remember that it is daunting but pretty fun to work with. You just have to keep it organized and check occasionally for desyncs! Luckily there is a nice method to test for desyncs yourself:
http://www.hiveworkshop.com/forums/miscellaneous-tutorials-456/multiple-warcraft-3-running-single-pc-226287/

Overall, good luck. I'll try to fish out my map if you can. This is a pretty fun system to make, but it can be frustrating.

Some links to have an idea about nice Inventory system:

http://www.wc3c.net/showthread.php?t=91163

http://www.wc3c.net/showthread.php?t=108895 <= this one seems to be the best option for you