Most Apache Isis viewers use icons to help identify domain objects in the user interface. It’s a good idea to ensure that these are styled consistently. Here are some open source icon sets for both personal and commercial use (though some require a link back to the authors website):
icons8 (as used in the Estatio app) - styled for Win8, Android and MacOS
tango desktop icon set - as used in Linux GNOME and KDE desktops
the noun project community of icon providers (both free and commercial)
flaticon - free vector icons
batch (300+ icons)
If you know of any others freely usable and easily accessible, let us know through the xdoc:../../support.adoc#[mailing list].