SteamPrices.com compares game and package prices from the Steam Store. At the moment, USD, GBP, EUR, RUB and BRL for a bunch of different regions are supported.
The site was created in order to show the huge difference in Steam Store prices for single games and packages. You can choose one of the supported currencies and compare "your" price to the ones in the other countries and regions. By and by, some comfortable comparison features and other stuff have been added, especially for registered users.
With the help of SteamPrices.com, you are able to search for games and packages and access multiple listings. Additionally, current spotlights and Steam news are also displayed.
Although you most likely cannot purchase games in other Steam Stores, you probably want to see if you could save some money. Maybe you know somebody in another country who you can trust in. He could help you by gifting the game. This is possible and legit. But please be aware of scammers.
A short note for the easily angry people: Don't blame Valve, they don't make or force the prices.
Don't hesitate to write an e-mail or ask in the Steam group if you got any questions, but please read the FAQ before.