Chem addict

As the name implies, chem addicts are characters who are addicted to chems.

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 and you will lose karma for doing so. However, you can avoid getting caught by simply closing the door behind you. 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).

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 this he replies he has nothing else to do but get high.

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

Unnamed addicts
Chem addicts who have gone insane with their chems and are hostile to the player can be found predominately around the El Rey motel. One is found in one of the rooms of the second level of the El Rey Motel. The others in the El Rey are dead, often lying on the floor.

Two non-hostile chem addicts are found at a camp fire just south of Fields' Shack across the railroad tracks.

Bill Ronte (Freeside)
Bill Ronte used to manage and maintain the Freeside water pump, but then became addicted to Dixon's whiskey.

Jacob Hoff (Freeside)
Jacob Hoff ironically brewed detox chems before he worked for the Garrets, and later becoming addicted to Dixon's Jet.

Fiends
All Fiends, with a few exceptions, are considered crazed druggies and are often in possession of lots of psycho, jet and med-x.