Papirus is a free and open source SVG icon theme for Linux, based on Paper Icon Set with a lot of new icons and a few extras, like Hardcode-Tray support, KDE colorscheme support, Folder Color support, and others.
Tela Icons
A flat colorful Design icon theme.
Kisary Remix Flat
Flat Remix ICON theme is a pretty simple Linux icon theme inspired on material design.