German developer Jürgen Nagel hosts a site with usefull ressource on the Settlers 2, unfortunatly only in german. It contains the so calld "FANpaign", a fan-made campaign with higher diffculty than the original one. There is also a second fan campaign "called 2ndpaign". Both replace the Roman Campaign in the original game.
Also, there are new maps and utilities. S2Utility in particular, which is basically a savegame editor and lets you manipulte your savegames to add ressources, harbour-spots and a lot mor. Unfortunalty, it's written in 16bit delphi, so it doesn't run on modern, 64bit OSes. I use it in virtual mashine, powered by Win98SE and Virtual Box.
http://www.jnsoftware.de/Siedler2/download.htm

There was also a great site of Sven Liebich (www.siedler-welt.de for those who remeber), but it has gone offline. It had a huge archive of custom maps, which I'm pretty sure I've downloaded them all. I haven't found them in my own archives, I'll upload them as soon as I find them.

Thank you for the link.
I remember Sven Liebich's site. It's a pitty that it's down now.

Pulling this very old topic from the graveyard because I stumbled upon the files I was talking about in the opening post.
I mirrored them at my webspace, including a dump of Sven Liebichs old website:
http://sok4r.de/temp/s2/ (ignore the stuff at the top, the script has difficulties handle some of the files and I haven't figured out a solution).

Sven Liebichs Website; http://sok4r.de/temp/s2/www.siedler-welt.de.7z / http://sok4r.de/temp/s2/www.siedler-welt.de.zip

Enjoy!