Proposed Ubuntu Splash screen. Damned funny.
(Not so much if I have to explain the joke).

While looking for a link to explain the joke above I found some parodies of those motivational style posters, describing the various distributions:

Ubuntu

#

Red Hat

#

Fedora

#

Debian

#

Slackware

#

Caldera

#

Mandrake

# (Mandrake now known as Mandriva).

Gentoo

#.
See also Gentoo users are Ricers which compares Gentoo users to modified car enthusiasts.
(the # symbol links to where I found the images, it would be inconsiderate to waste their bandwidth by direct linking them)