Assuming you haven't rebound it, while playing a game hold down the Shift key and then hit the Tab key, that will open up the Steam Overlay which gives access a number if features. A lot of them could be accessed by Alt+Tabing but some things like in-game notes can't.

Originally posted by Glimmer:

A lot of them could be accessed by Alt+Tabing but some things like in-game notes can't.

A number of games don't react well to alt+tabbing. The shift+tab overlay provides a safer alternative to access Steam features while in game. Although, there are some games that don't work well with that, either.