Preview different types of files with a single key
I've been playing around with Apple Mac OS X Leopard and, although the system didn't impress me that much, I loved one feature that I'd really like to see in Ubuntu.
It's call Quick Look, and allows you to view the contents of a file without open its default application.
This is great when you have a folder with different types of files (JPG, MPG, DOC) and you want see the content of the files quickly without open many applications (Eog, Totem, OpenOffice Writer in this case)
This is how it's works: you selected a file and press a predifined key (on OSX it's the spacebar) and it shows an instant preview of the file. This way you can preview all your documents (or any other file) and open only the one you need.
This program could use plugins for add support for different types of files.