A collection of modern icons for your applications. Licensed under LGPL.
Monochrome action icons
Cuttlefish is the app helping you find the perfect icons for your applications.
KDE Human Interface Guidelines
The KDE Human Interface Guidelines (HIG) offer designers and developers a set of recommendations for producing beautiful, usable, and consistent user interfaces for convergent desktop and mobile applications and workspace widgets. Their aim is to improve the experience for users by making application and widget interfaces more consistent and hence more intuitive and learnable.