Sushi is a new GNOME 3.2 tool similar to Gloobus Preview that provides quick file previews. To use it, select a file in Nautilus and then press the SPACE bar.Sushi works with both Unity and GNOME Shell and it actually doesn't depend on Nautilus so other file managers can make use of it.It is capable of previewing documents, PDFs, video and audio files.
Install gnome-sushi on ubuntu 11.10
Open the terminal and run the following command
sudo apt-get install gnome-sushi
After installation you need to logout and login back