steamcalc

SteamCalc allows you to keep track of your Steam purchases. It tracks the cost and time you put into each game and presents everything in tabular and graph form. You can see which games you have not yet played, which games have cost you way more for the same amount of playtime and even how much money you've spent on games you have never played. Easily find games that you have neglected to play before. You can also choose to share and compare your purchasing habits with your friends.

If you are new to steamCalc you might want to start by parsing your Steam store transactions on your profile page, just try the demo or take a look at this example. Report bugs by adding wl on Steam.

21 Nov 2015 update:

18 Nov 2015 update:

Filters
Columns

This graph plots the cost and hours of all bundles into one graph. Games below the average line have a worse cost/hours ratio.

9.999.99

BETA: To automatically parse your store transactions, you have to open Steam and go to My Account (top right) -> Account Details. To view the source of a page, right-click and select View Source...

Paste the source code from View purchase history into the first box. You have to click Load more transactions to get them all.
Paste the source code from View licenses and product key activations into the second box.

Parsing may take several seconds depending on the number of transactions and games in your account.