Well, I bought Neverwinter Nights in the Winter Sale, so now I'm just wondering - seeing as Gamespy closed down, how do I get it it to work with Online Multiplayer? Yes, I know that that pretty much makes me The Biggest Twat Who Ever Walked Amongst The Earth™, especially with so many alternatives to get NWN Online working, and so many guides to get them to work, but I'm new to the world of modding and patching, so I get confused by this kind of stuff.

Any help with this would be greatly appreciated!

You need to request your unique CD key from GOG support, and then you need to know the address of the server you want to connect to...

1. Make sure every player uses a different key. As Crispy78 said, you get one from support:
http://www.gog.com/en/support/contact/technical_issues_with_games/neverwinter_nights_diamond_edition

2. Bypass the account authentication:
You have to edit the local "Hosts" file.
It is located here: C:\Windows\System32\drivers\etc (Win 7 32bits)
The file is supposed to be without any extensions.
You might have to run the explorer as administrator to be allowed to open and change the hosts file.
- make a backup copy
- open the "hosts" file with notepad
- add these two lines:

127.0.0.1 nwmaster.bioware.com
127.0.0.1 nwnauth.kr.infogrames.com

- the lines must look exactly as above. No character can be before 127.0.0.1
- press Ctrl+S in notepad to save the file
- check that there's only a "hosts" file and no "hosts.txt" (in case, delete the "hosts" and rename the "hosts.txt" to "hosts")

3. To host the server you need to host it in game (The dedicated server application isn't working properly).
Ports to open on the server's host:
- 5120 through 5300 (Depends on how many players will play on your server. One port per player.)
- 6500
- 27900
- 28900

It worked! Thank you so, so much! (and thanks to Crispy78 and DeMignon for suggesting solutions as well) :D