There are quite a few maps, campaigns and also editors to be found on the internet, but sadly some links are dead or hard to find. Some maps are modified versions and not just a single player map, but those which actually provide harbors (which cannot be done with the map editor provided by bluebyte).
In order to install those map packs, make sure to do the following
$ cd mapname $ cp *.das *.DAS *.exe *.EXE ~/.dosbox/drive_c/bluebyte/siedler2/save $ cp *.lst *.LST ~/.dosbox/drive_c/bluebyte/siedler2/data $ cp *.ger *.GER ~/.dosbox/drive_c/bluebyte/siedler2/data/txt
Now get onto your dosbox and replace current save slots with the copied .das saves (this is necessary in order to load the modified harbor positions)
$ dosbox cd bluebytesiedler2save dirinst.exe
Mark the new map on the left, use tab to change to the right, select the target, and hit return to make it happen.
The campaigns normally would overwrite existing data, so make sure to keep backups.
$ cd ~/.dosbox/drive_c/bluebyte/siedler2 $ zip -r data-backup.zip data/ $ zip -r save-backup.zip save/
After that, copy the files manually – and keep in mind that linux will recognize case sensitivity while the files where originally saved on windows (make sure not to have duplicates in there).
In order to use the editors, place them either in main directory (c:bluebyte/siedler2) or one down below into the save directory. Starting is the same as the main game, the inner menues are rather self explainary.
Further resources:
http://www.muenster.de/~niki/
http://jh-isch.de/s2/seiten/mapdownload.php
http://jnsoftware.de/Siedler2/download.htm
http://mitglied.multimania.de/siedler5/sied2map.htm
http://jh-isch.de/s2/seiten/downloadhafenpack.php