As of Notes 8.5.2, application developers can specify an image resource as the database (application) icon. (You do this in Domino Designer. Kathy Howard explained how to do it in this blog posting on the Notes Design Blog. The icon shows up in the Workspace, on the Open list, and in the Tab.
You are free to design and use your own, but here on OpenNTF, we are happy to provide you with some. The ones on OpenNTF include icons for Mail, Contacts etc. -- that is , a few of the icons that we were unable to update in the Workspace in Notes 8.5.2. So now you can do it and make all the icons consistent if you want.
We are also hoping that this OpenNTF project becomes a community resource where other can share their application icons.
Here are the ones from the first release of the project:
Mary Beth Raven is the project owner, Niklas Heidloff only the committer.