There are a few websites that allow for this price tracking:
Is there Any Deal
Compares the prices of multiple vendors and gives a lowest and regular recorded price from these vendors. It provides a useful way of knowing whether the steam sale is actually worth it, or if other vendors have offered it lower.
Allows you to see a price history of steam games over a period of time. This is useful for knowing if the sale that is on now is actually a deal, or if it just seems like a deal because it's the holidays and it "should" be.