Given:
I am logged with different users simultaneously (admin and developer).

When:
I am using the application as any one of them.

Then:
I expect that I know immediately what user I am currently controlling.

Could be done as:
Simple label in some corner of the web page.

Reasoning:
After some time it is hard to recognize what user I am in that particular browser window. Even I am logged in as a single user, that information would be handy as well.