WarningSome people have allergies to latex, a form of rubber. If you do not know if you are allergic, a tiny amount should be tested first at minimum, or consult your medical professional.
Liquid latex is a staple of special effects makeup. Its simple basic application (paint it on) can provide seamless fake skin that can be peeled etc. It can be layered for thicker effects, used in substitution with Spirit Gum to apply prosthetics, and generally I like to use it as a sealer to get more life out of less durable effects.
Be sure that you are not allergic to latex before using this. You don't want to look like a zombie even after you remove the makeup. Here are instructions on how to check for allergies.
If you are allergic, for many purposes you can use Elmers/PVA glue as an acceptable substitute.
Always shave any areas before applying latex. You may not think you have hair on your toes/hands/cheeks etc. I assure you that you do, and it will pull them out at the root.
Although you can speed up drying with a hairdryer, on too high a heat this will cause yellowing and corrosion of the latex.
Tutorials using latex
You can often find small bottles in dollar stores in the run up to (and even cheaper, after halloween). Small amounts also often come in the $2-$4 makeup kits.
Beyond that costume stores will stock varying size bottles, bulk obviously being cheaper, and the online links below have different amounts that should suit any need.