manipulate X11 keymaps (for xmodmap) graphically

xkeycaps

manipulate X11 keymaps (for xmodmap) graphically

xkeycaps is a graphical front-end to xmodmap(1). It opens a window that looks like a keyboard; moving the mouse over a key shows what KeySyms and Modifier bits that key generates. Clicking on a key simulates KeyPress/KeyRelease events on the window of your choice. It is possible to change the KeySyms and Modifiers generated by a key through a mouse-based interface. This program can also write an input file for xmodmap to recreate your changes in future sessions.
http://www.nongnu.org/xmakemol/

xmakemol

program for visualizing atomic and molecular systems

XMakemol is a mouse-based program, written using the LessTif widget set, for viewing and manipulating atomic and other chemical systems. It reads XYZ input and renders atoms, bonds and hydrogen bonds.
X-based Quake Server Browser

xqf

X-based Quake Server Browser

XQF is a GTK+ frontend for Qstat, a tool for querying first-person action game servers like Quake, Unreal Tournament, etc. It displays information about the servers and allows configuring and launching some of the games from it.
Lode-runner-like platform game for X

xscavenger

Lode-runner-like platform game for X

XScavenger is a platform game for the X Window System. From the README:
Shisen-sho puzzle game for X11

xshisen

Shisen-sho puzzle game for X11

XShisen is a puzzle game for X11. Similar to the famous "Shang-hai", your objective is to remove all the pieces on the board.
Xtrlock cursor in locked mode

xtrlock

Minimal X display lock program

xtrlock is a very minimal X display lock program, which uses nothing except the Xlib library. It doesn't obscure the screen, it is completely idle while the display is locked and you don't type at it, and it doesn't do funny things to the X access control lists.