The Inspector in QuickTime Player!
I don't think I have ever talked about the Inspector in QuickTime Player, but I use it all the time. I do a lot of video and the Inspector in QuickTime really gives you all the info about what the file really is. You can also figure out if a file is playable on an iPod/iPhone by seeing the size and the data rate. It also displays info about frames per second and codec. The feature that I like the most in that Inspector is Current Size. I didn't really understand the point of the Current Size at first, but I now use it a lot. The way I use it is that I often work with HD files that I know will end up being further compressed and resized to a smaller size. I want to get a preview of what the file with look like. I really just have to resize the open QuickTime window and the current size will update with the info about the size at which it is displayed on screen. You can access the Inspector through the keyboard shortcut Command - I.