If you have $50 and a few hours to spend you can:
Switch regular light bulbs for CFL's
Install Energy Star qualified programmable thermostat.
Add weather stripping and foam insulation.
If you have $100 and a day to devote you can:
Install an Energy Star qualified ceiling fan.
Use motion detectors for outside lights.
Purchase Energy Star light fixtures.
If you have $200 and a weekend to spend you can:
Create an "envelope" around your house.
Replace single pane windows.
Ventilate your house with a whole-house fan.