From Team Fortress Wiki
backpack.tf is a community-created website developed by Steam user Brad Pitt for use as a price guide for Team Fortress 2. backpack.tf supports individual prices for niches such as Unusual effects and non-craftable items.
The regular pricelist lists all items that can be traded with columns for the various item qualities. The Unusual pricelist is kept separate from the main pricelist to account for the pricing of individual effects. Any recent changes to an item's price are indicated by a colored arrow. A Red Arrow going down means that the price has been lowered, a Green Arrow going up means the price has risen and a Grey Star means that the item has just got it's first price.
Unlike traditional price guides, backpack.tf allows users to submit their own price suggestions which then get voted on by other users. Correctly submitting or voting on item suggestions builds towards a user's on-site reputation. Active and accurate site users are recruited as staff to moderate positively-voted submissions to prevent any malicious price rigging.
backpack.tf also serves as a backpack viewer and uses its own price system to calculate the value of a user's backpack. When pricing a user's items, attributes such as an item's tradability, craftability, level, paint and craft number may be taken into account to modify the base price. Backpacks can be sorted and filtered in numerous ways, such as being ordered by price.
Historical data of backpacks are also archived, including a graph of a backpack's value and number of items over time.
backpack.tf has released a public API documented here. The API currently has two modes, one for user statistics including backpack worth and one for complete pricing data used by the site itself. Various third-party projects use backpack.tf's API, such as TF2 Warehouse and TF2Bazaar.