Reflectors are a basic tool, and can be useful in a variety of situations. Foamcore from the hardware store, auto sun shades (foil or matte, for a different effect), or even white cardboard will all work for reflecting light. Keep one or two of these handy.

Clamp Lamps

Clamp lamps are a good low cost light source that can be purchased on the cheap at almost any hardware store. Bulb wattage can be varied to control the intensity.

Work Lamps

These are a heavier duty light source than the clamp lamp. They are relatively inexpensive, available at most hardware stores, and some even come with tripods.

Chinese Lanterns

Also known as chinaballs, these cast an evenly diffused soft light. Get them in chinatown, at many box stores (ikea, target, etc), in an import store (possibly expensive), online, and even in some professional video/photography shops. Use extreme caution when ligthing with a Chinese lantern -- they are flammable! Usually you need to get the light socket and bulb separately (100-200 watts).

You can tone down a harsh light by diffusing it with a piece of fabric. You'll need to find some white silk or translucent nylon (ripstop works well). After that, you must figure out a way to safely place the diffusion panel between your light source and your subject.

For details, see Light Diffusion

The internet has a ton of awesome resources for Do It Yourself lighting. You can find tricks, short-cuts, cool equipment, and other low cost techniques to make your video look professional.

For details, see DIY Lighting