I would have to say that one of the longest-running jokes in gaming has to be Duke Nukem Forever. The game has been in development since sometime in the 1850's and will be released whenever 3D Realms is good and ready. We've seen hints that the game is still being worked on and is getting somewhere near completion, but nothing solid. The guys over at ShackNews, however, have actually seen the game.
Okay, so they haven't seen the completed game, because as you would imagine, it's not done yet. They did see the current build of the game including several different DNF gameplay scenarios, mechanics, and environments being demonstrated.
Apparently they were rather impressed with the game and seem to think that it will stand out visually from the rest of the crowd. Of course if history repeats itself, what they saw will soon be scrapped and work will begin anew on the title.