Tango/GNOME Icons Wikipedia pages (previews many but not all icons): https://commons.wikimedia.org/wiki/Tango_icons https://commons.wikimedia.org/wiki/GNOME_Desktop_icons
Breeze is a bit more 'modern' and would also work well on HiDPI: https://github.com/KDE/breeze-icons ; you would probably want to clone the repo and browse the files manually since GitHub isn't great for previewing images.
KDE created/used the Oxygen icon theme before Breeze, its style is between Tango/GNOME and Breeze: https://github.com/pasnox/oxygen-icons-png