The Tango icon library proposes both an interesting naming scheme and a coherent iconset.

First ... I like the iconset, but it's distributed under a cc-by-sa-2.5 license, so I sent an email to the FSF "licensing hotline" to ask if it could be included in Moodle. I was just told there is no problem including icons under this license in Moodle. So, good news for themers : we can use Tango icons. The iconset comes with 16x16, 22x22, 32x32 and scalable images.

Second, the Tango naming scheme is now used in xfce, gnome and kde. If we used the same naming scheme, we could (easily?) use icons made for these frameworks (among others - openoffice, pidgin, gimp and others also use either the same icons or the naming scheme). This would require significant work to move/rename each icon (there are 229 icons in Moodle 1.9).

Tango project: http://tango.freedesktop.org/Tango_Desktop_Project
The iconset can be seen on http://tango.freedesktop.org/Tango_Icon_Library (files are available by cvs)