Extend the lives of filters in some air purifiers, bathroom fans, window air conditioners, and even your furnace by vacuuming out dust and loose dirt. Just be sure your vacuum is HEPA equipped to keep fine particulates from being released into the air.
Savings: About $40 for two replacement air-purifier filters. You typically have to change them four times a year, but thorough cleaning can cut that down to just two times.
Bonus: While you've got the vacuum nozzle hooked up, use it to suck up dust bunnies and spiderwebs in hard-to-reach places.