Actually all spiders are venomous to insects, that is how they kill, by injecting venom into their prey, but most spiders either do not have fangs that can penetrate human skin or else the venom is too mild to affect us. It can be like a bee sting where one person isn't affected beyond the pain of the sting and another has an allergic reaction to the venom of the sting.

The true daddy long legs, the first one, is not a spider but is actually considered more of a mite. It has only one body segment (thorax) whereas spiders have two, head (cephalothorax) and thorax. They do not have the ability to spin silk at all.

I'd guess both are commonly referred to by people as daddy long legs and Mythbusters used the real spider (the second one pictured) for their show.