Now that L4D2 came out, enter these in console when haveing SV_CHEATS 1 on;
ent_create weapon_smg_mp5 OR give smg_mp5
ent_Create weapon_rifle_sg552 OR give rifle_sg552
ent_create weapon_sniper_awp OR give sniper_awp
ent_create weapon_sniper_scout OR give sniper_scout
They won't be able to do any damage, though keep them untill the next level comes up, then the game will be able to register the damage data and your done!
Bugs-
Surivors dont say anything, and sometimes don't pick them up. I don't know about the knife weapon. Sorry for the bump, anyways.