Since I got this question today I’ll just write up an easy answer for it here, for those of you who didn’t follow the Windows Blue (8.1) release. Peek to definition is a new function in Visual Studio 2013 (now as a preview) that allows you to have a peek at the implementation of a type or member in a smaller popup window where your cursor is instead of having to navigate to the implementation by using F12. This is something I most certainly will use a lot.
To use it you simply leave the cursor at the item, and the hit Alt + F12.
Esc will close the peek.
This is available in C#, VB and C++.