Web Spinners

The first method is done by bolting a can to a fan blade, the can has a few holes drilled on the walls near the top, fill it half way with glue, lock the bolt in the chuck of a drill and spin the unit very fast forcing the glue to come out of the holes, which the fan blades blow toward your target.

Advantages:

Extremely realistic and life like webs are possible.
Glues that are less aggressive are available.
No compressed air required to apply the webbing glue.
Glue that is less aggressive is easily removed from many surfaces after your haunt is done.
UV additives are available for your glue.

Disadvantages:

Glue vapors can be toxic.
This method cannot span extremely large gaps between props.
Requires electricity to operate.
Most do not come with a safety blade guard.
Does not operate well in windy conditions.
Easily destroyed in bad weather.
Some glue can be highly aggressive.
Spin all your webs at one time as the units can clog quickly. You can help minimize this by placing masking tape over the holes when not in use.

I would also like to point out that the glue for this type of system is extremely flammable. Most glues are flammable in their vapor, liquid or cured forms. There are some glues that are not considered flammable once dry.

What to look for in a quality unit.

7" blade diameters (maximum) with blade angles of 24 - 27 degrees for units that are going to do detail work. This is great for detail work and to add a little UV webbing to existing webs you shot with the larger systems.
8" - 10" blade diameters with blade angles of 36 - 40 degrees which are great for area coverage.
Blades designed to rotate up to 3200 RPM minimum. While you will spend most of your time spinning webs at approximately 1500 RPM to 2000 RPM most drills used are designed to spin at up to 2500 RPM. Lower quality web spinner manufacturers will specify 1750 RPM as the maximum speed due to the fact that the cheaper blades are not designed to withstand higher amounts of force.
Glue reservoirs of 4 ounces for detail work.
Glue reservoirs of 8 ounces for coverage work.
Bolts that are grade rated steel.
5/16" blade hubs versus the cheaper 1/4" blade hubs. 5/16" blade hubs typically only come on fan blades designed for a maximum rated RPM of 3200. The lower quality blades come with a 1/4" blade hub as they are not designed or built to rotate at speeds higher than 1750.