Believe it or not, but I decided to pick up Silent Hunter 3 for a good price a week ago....that's right, the submarine sim series.
Contrary to what one would expect, it's pretty darn awesome fun.
The graphics are ok (well 90% of the time it's just open sea anyway), but the atmosphere...omg! It's just great.
You can opt for as much realism as you want, but the standard mode has the AI taking over quite a few tasks (thankgod). The sub is a huge collection of gauges, options, stations, signals, etc.
There's a career mode that lets you start as a lieutenant in a crappy sub at the beginning of the war, 1939, when the allied fleet was still pretty defenseless. You get assigned patrol zones, but you are (relatively) free to roam the seas in search of prey. As you sink ships, you gain in renown and are able to recruit better crew/commandeer better subs.
It's hard to describe the feeling of satisfaction you get after a clean, quick attack...pick up sounds/signals, plot out estimated collision course, dive to periscope depth and aim...and then disappear before the convoy even realises the torpedo hit.
Last mission I managed to enter the allied port of Hartlepool undetected...I started diving from a long way out, alternating between low speed (to minimize chances of sounds being picked up by ships near) and shutting off the engines entirely when an allied military ship passed by. When i came in the actual docks, I went to periscope depth and sunk a merchant and a cargo ship in its berth in a space of 2 minutes. They didn't know what hit them.
And if it weren't for me running out of torpedos, I might have sunk another tanker as well (didn't have a deck cannon and flak wasn't doing shit on the thing). I made my getaway as cruisers rolled in from all over the area, though I did lose my sonarman through an unlucky depth charge hit on my radio quarters. 
Erh...sorry. It's just great fun. I changed the speech in-game to German, too. Torpedos los!! Seriously, nothing breaks immersion like English-speaking crew on a U-Boot.
