Clamp Light

The clamp light is the best unintentional invention for film making. They can be found at any hardware store and typically cost about $8. They are essentially a 10 foot cord with a light bulb socket on the end. You can keep or remove the metal reflector dish depending on the kind of light you want. You can put any kind of standard bulb in them as long as it is less than about 500W. I find that 23-45W CFL lights work best because they produce much more light than an incandescent bulb, but if you're on a tight budget you can always grab regular bulbs.

The best place on the internet to find all kinds of bulbs is 1000bulbs.com

Tip: The "standard" socket is called E26 or E27. They are pretty much the same thing.