Below is a sample repository with sources I have found useful. #'s will symbolise comments and I will provide the keys where possible:
# Avant Window Navigator
deb http://download.tuxfamily.org/deb4gilir/avant-window-navigator/ unstable main
# Compiz Fusion
deb http://download.tuxfamily.org/shames/debian-lenny/desktopfx/unstable/ ./
# Multimedia (w32codecs, libdvdcss)
deb http://www.debian-multimedia.org testing main
# VirtualBox
deb http://www.virtualbox.org/debian etch non-free
