I've seen newbs on defense in Battlefield: Bad Company use grenade launchers to destroy all cover the enemy might use while attacking. If you can't shoot straight, it's useful to destroy any walls they might cower behind. You can also create cover for teammates by digging foxholes with grenades.
In Battlefield: 1942, there were some unusual ways to help your teammates. There was the "mortar crew", a strategy where 4 guys use bazookas and ammo crates to constantly lay down small-scale artillery fire. It worked well on Tobruk for the Allies, and to a lesser extent on Omaha Beach for the Axis.
In the Battle of Britain, each side could try some unusual strategies. British pilots could try to fly over to the German airbase and steal a bomber, flying it in a holding pattern to deny the plane from German control. The German team could try filling a bomber with anti-tank units, and dumping all of the bombs and passengers over the tough-to-destroy British factory. These units proceed to destroy the factory with their anti-tank rockets.
It was also possible for the British team on that map to give up on defending most of their buildings, and use Engineers surrounding the tough factory to repair it constantly. This strategy is tough to organize, but it was generally considered very unfair.
