Gloobus is an very cool extension to enable a full screen preview of any kind of file.You can quick preview your folders,PDFs,video/audio,pictures with just press on space key.

Currently, Gloobus supports:

Source: C++, JAVA, PHP, SH, PYTHON,
Images: JPG, PSD, XCF, PNG, GIF, SVG, ICNS
Documents: PDF, CBR, CBZ, DOC, ODF, XLS, ODS, PPT, ODP
Music: MP3, OGG, MP4, MIDI, WAV
Video: MPG, AVI, OGG, FLV, WVM, MKV
Other: Folders, Plain text files

Install Gloobus

For ubuntu 9.10 user:
open terminal from Applications/Accessories menu,add the repository:

sudo add-apt-repository ppa:gloobus-dev/gloobus-preview

update,install gloobus and upgrade to the patched nautilus:

sudo apt-get update
sudo apt-get install gloobus-preview
sudo apt-get upgrade

restart nautilus by:

nautilus -q
nautilus &

Alternatively,download the deb package from http://gloobus.net/download/
ScreenShot:
Now,choose something in file browser or on the desktop that you want to quick preview.Press on space key to preview and press this again to stop preview.
quicklook

quicklook1

For more,see:https://launchpad.net/gloobus