Gloobus-Preview is a Nautilus extension that allows Ubuntu users to preview videos, audio files, images, folders, and many other file formats without opening them. You simply select a file and press the space bar to preview it, it has the same functionality as Gnome Sushi.
Gloobus-Preview now supports Nautilus 3.2 and can be installed on Ubuntu 11.10/11.04 with these commands from the terminal:
sudo add-apt-repository ppa:gloobus-dev/gloobus-preview
sudo apt-get update
sudo apt-get install gloobus-preview gloobus-sushi
Use now the space bar to preview any file or folder on Ubuntu: