Ever wanted to know how long can you keep the ketchup in the fridge or when does your Pepsi go bad? This site has a pretty comprehensive list of Food and Household products and how long they last. The most surprising thing is mayonaise, according to the site:

Mayonnaise
Unopened: Indefinitely
Opened: 2 to 3 months from “purchase by” date (After this time, color or flavor may be affected, but product is still generally safe to consume.)

For some reason I though mayonnaise goes bad very quickly, this is why I usually throw out the jar after the first use. Actually, anything that has been in my fridge for more then a week gets thrown out, unless its in the freezer. [Link]