[Sailor Steve]

Razark pretty much hit the nail right on the head.

RFB pretty much recreates the Pacific war.

TMO makes it harder, which draws some complaints, but the idea is that having it exactly as it was gives the player the advantage of hindsight, and making it harder actually makes you think twice about what might happen if you're not careful.

There is also GFO (Game Fixes Only) which, as it says, is dedicated to fixing the stock game.

And FOTRS (Fall Of The Rising Sun), which I know very little about, but you can find out a lot about it by reading the thread for that mod.

One of the things necessary in conjunction with the first three is the Run Silent, Run Deep campaign, which changes all the campaign files to reflect historical movements and battles. It's a great addition to the game.

Operation Monsun is the only SH4 u-boat campaign, and it's a good one. So good, in fact, that the only things that keep me playing SH3 are the graphics mods and SH3 Commander. OM doesn't have any of the cool harbor stuff, or GWX's blimps or LSH3's awesome Harbors and Kiel Canal trip, or Commander's crew and ship names. If those things were ever done for SH4 I would give up SH3 forever. That's how good it is.