The Grey Wolves addon for Silent Hunter III. It's a ground-up rewrite of the career campaign, which adds oodles of new ship-types and sorts out the historical timeline that was only vaguely accurate in the vanilla game. Countries enter the war at the correct times, and access to ports and supply ships change in real time, not in between patrols as previously.
It also counts as a bona-fide addon, as it adds the Indian Ocean as an operational theatre (should the player wish to use it) from about 1942-3 onwards, which was a massive omission from the original. New maps and charts that can be dragged out and viewed from the navigation map are provided that give you lots of info that wasn't in the original - safe routes through the minefields around U-boat bases, for instance.
Traffic is more varied too, meaning you'll rarely see two identically marked vessels in the same convoy. Neutral ships run with spotlights shining on their flags at night, neutral coastlines will be lit up and all the important phases of the Battle of the Atlantic are accurately recreated. The list of tweaks and improvements runs into the hundreds, at least. It really is a case of the game being almost unrecognisable from the original once you pop the mod in. It's also free, as it was made by fans.
It was so succesful that for the sequel, Silent Hunter 4 the dev team did the Indian Ocean addon as a separate commercial product, and didn't provide a fraction of the improvements that the fan-made mod for SH III did.
It's definitely among the best addons I've ever come across.