A nice and straightforward screen capture utility.

zathura is a highly customizable and functional document viewer. It provides a minimalistic and space saving interface as well as an easy usage that mainly focuses on keyboard interaction.

Zathura-plugin-MupDF extends the document viewing support of Zathura to PDF, EPUB and OpenXPS with the help of MuPDF rendering engine.

i3lock is a screen locker like slock. After starting it, you will see a
white screen (you can configure the color/an image). You can return to
your screen by entering your password.

Waffle is a cross-platform C library that allows one to defer selection of GL API and window system until runtime. For example, on Linux, Waffle enables an application to select X11/EGL with an OpenGL 3.3 core profile, Wayland with OpenGL ES2, and other window system / API combinations.

Waffle's immediate goal is to enable Piglit, Mesa's OpenGL test suite, to test multiple GL flavors in a cross-platform way. However, Waffle's design does not preclude it from being useful to other projects.