You can find a list of mods here:
http://www.subsim.com/radioroom/showthread.php?t=162013
I assume the reason that they aren't all on the SubSim download page is because you need to PM Neal to set up an account to put files up there, while most online file serving websites it's all automatic.
Might be size concerns too. I know for pictures here we have a very very small size and resolution limit for the albums while places like Imageshack don't.
Either way check that link I gave, it tries to collect all the various mod links.