Left For Dead Characters
It depends on what you mean by "modeled after real-life people".
If you mean visually, then yes! Valve usually models the face of their characters based on some real-life person (probably a hired model or actor, but I can't confirm this). From Left4Dead Wikia, here is a list of the face models for each character (as well as the voice actor):
| Game | Character | Face model | Voice actor |
|---|---|---|---|
| L4D1 | Bill | Bernard Fouquet | Jim French |
| L4D1 | Francis | Taylor Knox | Vince Valenzuela |
| L4D1 | Louis | Sean Bennett | Earl Alexander |
| L4D1 | Zoey | Sonja Kinski | Jen Taylor |
| L4D2 | Coach | Darnell Rice | Chad Coleman |
| L4D2 | Ellis | Jesy McKinney | Eric Ladin |
| L4D2 | Nick | Taymour Ghazi | Hugh Dillon |
| L4D2 | Rochelle | Shanola Hampton | Rochelle Aytes |
The body is easier to model in 3D, and is probably based on sketches made by Valve artists. However, it is very difficult to accurately model a face, and that's why they use real-life models for faces. (but, of course, they fine-tune the modelled face to fit better to the character, like the G-Man from Half-Life series)
It appears that they are just stereotypes:
Wikipedia article
There are four playable human characters in the game: Francis, a stereotypical biker (voiced by Vince Valenzuela); Bill, a Vietnam Veteran (voiced by Jim French); Zoey, a university student (voiced by Jen Taylor); and Louis, a district Account Manager (voiced by Earl Alexander).
(my emphasis)
However, I wouldn't be surprised if each characters appearance wasn't based on a real-life person - but that person could be anyone, even one of the developers!