I looked for the Workspace Icon ( a database cylinder ), but I could not find it. While inspection showed that it was part of the core.min.css. After an hour of searching I looked at the source files in Firebug. There are two versions of this css file, one in the apex_ui folder and one in the app_ui folder.
I have created a page where both collections of icons are listed:
The icons can be used in HTML by adding the classes a-Icon and the class of the specific icon (the text behind the icon on the page):
<span class="a-Icon icon-find"></span>
The Apex Builder icons can be used by referencing the file apex_icons.css. Put this file on your server or in the static files and reference it on the page with the icons.
Have fun using the icons,