These themes that you can download from OMGUbuntu.co.uk theme the toolbars in Nautilus and make it much more attractive. These are very good themes to use with the MATE desktop as well.
http://www.fandigital.com/p/zoncolor.html. Zoncolor themes for Ubuntu and MATE. These are very customisable themes for your Linux desktop.
Ubuntu Ambiance theme for KDE; this makes your KDE desktop look like the Unity desktop: http://half-left.deviantart.com/art/KDE4-Ubuntu-Ambiance-346399730.
Uncomplicated GTK 3 theme for MATE and Gnome 3: http://gnome-look.org/content/show.php/Uncomplicated?content=157986.
Elements GTK theme for Unity and Gnome 3: http://gnome-look.org/content/show.php/Elements?content=145920.
More lovely GTK 3 themes: http://www.securitronlinux.com/bejiitaswrath/lovely-gtk-themes-for-the-linux-mint-mate-desktop/.