Laser collimators are commercially available and almost all vendors offer the Barlow attachment described by Nils.While not necessary for accurate collimation (a Chesire accomplishes the same thing, really) these barlowed laser collimators simplify (some say oversimplify) collimation for Newtonian users, and without the barlowed attachment can help align the secondary and collimate the focuser itself.
In my case, I just taped some foam to the spring-loaded built-in clamp at the base of my Logitech laptop webcam, and attached it to a spider vane. I had to insert foam in such a way that the camera was angled toward the focuser.
Rather, you are just using the webcam software to project the image of the webcam onto your own computer screen. For nighttime use I have a red acrylic shield over my screen to preserve night-vision.
If you are going to use this technique in the field in the dark with other observers, you'll need some form of laptop screen shield to preserve everyone's night vision.
I do know users with large truss Dobsonians who use a finder scope to watch the laser (by its reflection in the secondary), or who use the Blug by Howie Glatter.
In truth, these are easier and cheaper ways to image the back of the collimator.
If you lack a shield, you should really do this only before dark.