A recent addition to this list would be a website known as trade.tf. This one is also a particular contender, since rather than relying on a community-driven system, like Backpack.tf, 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.
Even if you'd still prefer Backpack.tf pricing, those are also shown on each item's page, and there are more features on the site other than pricing. 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, along with an insta-trading system allowing you to automatically accept any trade offer meeting the criteria you set.
Note: Unfortunately, as the FAQ states, Unusual items aren't tracked on the spreadsheet. This is simply because unusual trades happen too rarely for the bot to take them into account. This may also be the case for Collector's items.