| [X11](https://www.x.org) | The X Window System (X11) is a windowing system for bitmap displays |
| [x264](http://www.videolan.org/developers/x264.html) | x264 is a free software library and application for encoding video streams into the H.264/MPEG-4 AVC compression format, and is released under the terms of the GNU GPL. |
| [x265](http://x265.org/) | x265 is a free software library and application for encoding video streams into the H.265 AVC compression format, and is released under the terms of the GNU GPL. |
| [XCrySDen](http://www.xcrysden.org/) | XCrySDen is a crystalline and molecular structure visualisation program aiming at display of isosurfaces and contours, which can be superimposed on crystalline structures and interactively rotated and manipulated. |
| [Xvfb](http://www.x.org) | Xvfb is an X server that can run on machines with no display hardware and no physical input devices. It emulates a dumb framebuffer using virtual memory. |