Here I have collected some awesome lists of projects I found very useful. Please check yourself if you can use them in your own project.

Art

These are nice artworks one can use.

Icons:

Material by Google. No introduction needed.
Feather. Very similar to the material icons, but a bit … lighter.
Breeze by KDE. Hard edges, thin lines and very good-looking. Used around their desktop environment, so there are a lot of icons to choose from.

Illustrations:

undraw. Nice corporate drawings free to use.
Old book illustrations. Title says it all: A collection of drawings found in books so old they are now royalty free.

FOSS App recommendations

Security:

Social Media: These are all services in the Fediverse, which means they are interconnected. Check them out!

Nextcloud Integrations for your smartphone:

Other stuff that doesn’t really fit into these categories:

KDE Connect. Connects your phone with your pc to exchange notifications, but you can also use your phone as a remote control, or the keyboard of the pc on your phone. Also adds media controls for the music and videos playing on your pc. Works for Linux, Windows and even MacOS.