Here is a list of five good working plugins you can use for suggesting Windows and Mac users about the advantages of Linux, and providing links to selected distros etc.
Ubuntu sidebar
A Ubuntu menu for your website, simple to install and setup, shows Ubuntu networking icons in a un-obtrusive way.
Debian sidebar
A Debian menu for your website, simple to install and setup, shows Debian networking icons in a un-obtrusive way.
Ubuntu ribbon
Help spreading Ubuntu by putting a ribbon on your WordPress site.
Debian ribbon
Help spreading Debian by putting a ribbon on your WordPress site.
Ubuntu sidebar lite
Another Ubuntu menu for your website shows Ubuntu networking icons in a un-obtrusive way.
Debian sidebar lite
Another Debian menu for your website shows Debian networking icons in a un-obtrusive way.
Linux Promotional Plugin
A SMALL popup with customizable message about the avaliablity of a free OS replacement for Windows, and Mac.