Non destructive editing in Aperture!
I know I talk a lot about Aperture right now, but it is because I use it more than ever and I discover a lot of cool tips that way. What makes Aperture stand out compared to many other editing applications such as Photoshop is because it is non destructive editing. What it really means is that the application always keeps the original copy and the adjustments are rendered on the fly as you use the application. It is one of the reason why Aperture is quite demanding in system resources. What I really wanted though was a way of comparing the original pictures with the adjustments made, similar to what you can do in iPhoto with "Shift". The iPhoto Shift trick doesn't work, but there is a button that does the same thing. It is in the lower right corner, the second one from the left. The feature is called "Show Master". You can also get the same feature with the shortcut "M".