One that hasn't been mentioned: No One Lives Forever 2's shotgun equipped with explosive shells. For extra fun, max out your weapons skill so you not only do more damage per shot, but you reload like crazy as well. *BOOM BOOM BOOM BOOM BOOM* *enemies go out in a flurry of flames*. The game has some simplistic ragdoll physics, so you can bounce bodies around with the shotty. Lovely.
Other than that the usual; Doom II double barrel, Halo, SoF, Half-Life alternate, Counter-Strike etc.