Greybird is the default theme of Xubuntu. The new version of the theme (3.20.0) now supports GTK 3.20 as well and it has been ported to SASS.
Adapta is a shiny new material design like GTK theme. It supports numerous desktop environments including Gnome, Budgie, Unity and Cinnamon.
A slightly modified version of the beautiful Arc GTK theme has been released with OS X styled window buttons.
Arc is a popular GTK theme. It now supports Cinnamon, Firefox, Google Chrome, Chromium and Plank dock.
There’s a new theme available for Cinnamon, called CinnXP which makes your desktop look like Windows XP. It could be ideal for people switching from Windows XP to Linux.
The beautiful Arc GTK theme is finally available in two dark variants as well: Arc-Darker and Arc-Dark.
Arc theme is a gorgeous GTK theme which looks and works well on Ubuntu / Linux Mint / elementary OS. Now it even got an official repository.