I'm glad to see that the new people are enjoying it!
The modding scene is VERY healthy, indeed. It's probably a little overwhelming, so lemme help out a bit.
The first thing you should know is this:
www.u-boot.realsimulation.com . This site has just about everything SH3 related, and more.
Some BIG PACK mods are out there as well.
Real U-boat (RUb) is a mod that attempts to make SH3 into the most realistic subsim out there. Get it here:
http://www.beerymod.com/
Improved U-Boat (IUB) is another big mod pack with realism at mind, but more emphasis on game playability (i.e. new menu screens, sounds, textures, etc.) Look for it here:
www.u-boot.realsimulation.com
And don't forget to check out the mod thread:
http://www.subsim.com/phpBB/viewtopic.php?t=30638 . It is constantly updated, so you'll find a bunch of neat stuff in there.
This is just the tip of the iceburg. There's more than a ton of things for you to try out. I hope this helps. And welcome to the community!