COD4 provides a large number of attachments to weapons.
Grenade Launcher: Can be fitted to assault rifles, although does use up the perk 1 slot. Also known as the Noob Tube as it can be used to gain easy first kills, although in the right hands can be used skillfully can prove to be extremly versatile.
Silencer: Can be fitted to assault rifles, sub machine guns and some pistols. Hides you from showing up on the map when firing, although does reduce your weapons range. However on some rifles you can still achieve extreme range shots accross the map with no problem so experiment (G3 😉 ). The silencer also reduces the guns recoil considerably.
Red Dot Sight: Usually better than any weapons default iron sites. Makes aiming much easier and quicker.
Grip: Light machine guns and shotguns, reduces recoil but you loose slot 1 perk.
ACOG Scope: Increases range of all weapons apart from shoguns, although accuracy is reduced slightly.