@Emilio: What are you using to do that? can you pull down the console in the game?
For the Left 4 Dead pics? Yeah, use the tilde key to bring up the console when I'm in a single player game. Console should be in the control options to turn it on., I believe.
I then enter >> sv_cheats 1
Then I enter >> kick bill; wait; kick zoey; wait; kick francis; (If I was playing as Louis)
Then I use noclip and zoomed to the location I wanted.
Then I enter "z_spawn; wait;" and I copy and paste that in one line, so I was spawning at least 20 zombies for every entry.
You can also do other stuff like z_spawn tank, or kick tank, and stuff like "give rifle" for weapons.