A recent addition to this list would be a website known as trade.tf. This one is also a particularly superior contender, since rather than relying on a community-driven system, the prices are determined through a bot that pulls in recent trades in the past 24 hours to determine the current price. Their FAQ states:
How does the spreadsheet work?
It all starts with a robot that polls user inventory changes from over 150,000 users.
First we determine what items were removed and what items were added
Second we clean the trade: in the added items, we check where they came from (drop, purchase, etc...) and eliminate the ones we know for sure are not coming from a trade
Third, we store the trade to be processed by the mathematical model
Every hour, the mathematical model is run on:
The trades from the last 24 hours
If there are not enough trades for a particular item in the last 24 hours, we use trades including this item for last 48 hours, or 72 hours and so on until there are a statistically high enough number of trades.
It also contains a search feature for trades across different trading sites and allows you to pick out good deals to make profit off of. This will also be covered in the FAQ.
Note: Unfortunately, as the FAQ states, Unusual items aren't tracked. This is simply because unusual trades happen too rarely for the bot to take them into account.