Windows: If you've ever played a full-screen game and wished you could open a browser or respond to an IM without tabbing out, Overwolf is the utility for you. The app adds useful tools so you can look up strategies, chat with friends and control your music, all without leaving the game.
Overwolf works like Steam's built-in overlay, but it's even less intrusive. You press a set of hotkeys or mouse over the widget on the side of the screen (you can see how this works in the video above), and a button appears with tools you can use.
In Overwolf's case, instead of getting what the service provides by default, you can choose the apps included. Overwolf has its own mini "app store" that has customised tools for Gmail, Skype, Facebook, IRC, and even streaming music services such as Pandora and Grooveshark.
There's even a full web browser, and mini-apps for specific games (a crafting advisor for Team Fortress 2, a talent calculator for World of Warcraft, timers for DOTA and a Battlelog viewer for Battlefield 3) that can help you out in-game.
The utility is free, and a Mac version is on the way soon. If you want to give it a try, you'll need .NET installed. You can check if your favourite games are supported here. Thanks Jordan!