Bind a key to sm_drop, or type in chat !drop (public trigger) or /drop (private trigger) to drop the weapon you are holding on the ground for another survivor to use.
If you allowed dropping arbitrary weapons, put the weapon name after the command to drop it.
(1.1) - Version CVAR
(1) - Whether or not to allow a player to drop a weapon they are not holding.
Requested by LTR.2 - http://forums.alliedmods.net/showthread.php?t=103818
1.1 - Because so many people requested it, you can now drop an arbitrary weapon without having to be holding it.