GWX is probably the most popular, LSH2015 is the newest, and WAC5.0 is in development. People also like WAC4.1 and NYGM. Those are pretty much the mega mods that come to mind.
Ahnenerbe's Widescreen and Makman's Magui (also has widescreen) are good UI mods and seem to work over all of the megamods.
Also, you specified 'free'. To my knowledge, all of the mods that you'll find for the Silent Hunter series are free. The modders have major sized hearts.
