There are some lovely MDM themes here: http://gnome-look.org/index.php?xsortmode=high&page=0&xcontentmode=150&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. These would be perfect for any Linux Mint 15 machine. The MDM login manager in Linux Mint 15 has the ability to load GDM themes as well as new animated HTML5 themes. These themes are perfect for any user that wants to customise the login manager.
This theme: http://gnome-look.org/content/show.php/Blue+Swirl?content=30846&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. Is really nice. It resembles a Fedora theme.
Here is a UAC themed login interface: http://gnome-look.org/content/show.php/SystemAccess?content=52145&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. This would be perfect for a Doom fan like me.
A lovely Ubuntu styled login manager theme: http://gnome-look.org/content/show.php/nUbuntu-Face?content=92786&PHPSESSID=677369a539b682bfcfaf4deb6bad7148.
Here is another Ubuntu styled login manager theme for MDM: http://gnome-look.org/content/show.php/Arc-Colors+GDM-Walls?content=88305&PHPSESSID=677369a539b682bfcfaf4deb6bad7148.
A simple and squared MDM theme: http://gnome-look.org/content/show.php/Simple+Ubuntu+GDM?content=95984&PHPSESSID=677369a539b682bfcfaf4deb6bad7148.
Debian Lenny theme: http://gnome-look.org/content/show.php/Debian+Lenny+List?content=93621&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. This is a Debian Lenny inspired theme for GDM/MDM.
DM Intrepid GDM: http://gnome-look.org/content/show.php/DM+Intrepid+GDM?content=91018&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. This is a beautiful GDM theme.
Wooden GTK theme: http://gnome-look.org/content/show.php/Wooden?content=90685&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. This is a theme that uses a wooden background and looks like paper pinned to the wood with the login prompt on it. Very nice indeed.
Relaxing GDM theme: http://gnome-look.org/content/show.php/Relaxing+-+Open+Solaris+edition?content=86350&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. This is a very professional GTK theme.
Debian Orange GDM theme: http://gnome-look.org/content/show.php/Debian+Orange+GDM+Theme?content=56433&PHPSESSID=677369a539b682bfcfaf4deb6bad7148. A nice orange GDM theme with Debian branding.