Chem addict

As the name implies, chem addicts are characters who are addicted to chems. Knowing these people is a good way to get some chems for yourself.

Fallout 3
There are quite a few addicts in Fallout 3.

Leo Stahl (Megaton)
Talking to the doctor in Megaton can get him to reveal Leo's crazy chem addiction. Mostly Jet. After this you can talk to him and talk him into quitting (good karma). He will give you a key. Go to the Water Processing Plant, and there is a locked desk full of Jet and money (along with some other chems).

Paulie Cantelli (Rivet City)
Talk to Vera Weatherly and ask her about some gossip. She can tell you some interesting stuff. Mister Lopez is considering suicide, for example. She can also tell you :that Paulie Cantelli is addicted to Jet. Asking around confirms this. His room is filled with Jet, Psycho, and other chems. It is not free to take, however, and you will lose karma. You can avoid getting caught by simply closing the door behind you, though. While speaking to him you have the option of offering him :Psycho. If you give him enough Psycho you will soon find him dead (see Overdose (quest)).

Sierra Petrovita (Girdershade)
Sierra Petrovita in Girdershade is addicted to Nuka-Cola.

Snowflake (Underworld)
When talking to him, he tell you he wants to cut your hair. Then a dialog option is available: "Are you taking Jet over here?", to what he replies he has nothing else to do but getting high.

Zip (Little Lamplight)
Zip in Little Lamplight is another Nuka-Cola addict.

Narkomane

Fallout new vegas
Chem addicts are people who have gone insane with their chems and are hostile to the player. One is found in one of the rooms of the second level of the El Rey Motel