What's the effect of crouching in Alien Swarm?

It's not possible to crouch, it's possible to walk (default binding is shift). When stationary and walking, the animation will show the marine crouched.

The purpose of walk is to be able to move slower when necessary (a rarity in my opinion), and also the minigun is constantly spinned when walking, just like if the alt fire is pressed. Oh, and it looks cool. That's about it.

All this is assuming you don't mean the defensive roll maneuver, which is something else completely, and allows you to evade projectiles (both enemy's and ally's).

EDIT: I've seen some posts in other forums that mention that "crouching" may improve accuracy (i.e. decrease the bullet spread). I've never noticed anything like this, and I browsed the game's source code and it seems that bullet spread does not depend on the stance.