What alternatives are there for teeth whitening? (Photo)

My teeth have been growing yellower the past year. At first i didn't think much of it, but it's been getting worse over time. It's really embarrassing and I can't pinpoint the cause or do anything to stop it. Brushing my teeth hasn't had any effect whatsoever. I don't think its my diet because I don't drink soda much, never drank coffee, don't drink, smoke, or do drugs. It's a bit frustrating. I don't have the money to go to a dentist yet, so any alternatives from a professional would be great.