X Server, allowing you to display <a href="http://en.wikipedia.org/wiki/X_Window_System">X11</a> applications on your device. It could be used in conjunction with https://f-droid.org/packages/info.guardianproject.lildebi to use graphical apps according to the instructions on <a href="https://web.archive.org/web/20141008132408/http://j05hyyy.blogspot.ca/2012_09_01_archive.html">this blog</a>