In early 2015 our split Question/Tags/Users/Badges button section joined the Ask Question button in aligning right. careers 2.0 finally tried to gain some cache by rebranding using the Stack Overflow name.
Edit: And, as mentioned by Armatus in the comments, note the Stack Overflow logo reduction of size and also angle of stack elements - with the highest element going to something like 55° from 80°. As a result of that investigation, I noticed that the sprite sheet for the site changed: old vs new. (capture 2015-02-01)▼
Let the notifications commence! The Inbox and Recent Achivements icons arrive in the menu bar (another sprite sheet). (capture 2015-10-14)▼
stack overflow careers departs the menu bar to become a gray Jobs button. The Unaswered button departs. Consider that at this point we're quantifying that Stack Overflow is composed of over 4.7 million programmers. (capture 2016-01-31)▼
The Developer Story + Documentation Beta. (capture 2016-10-31)▼
Beginning in 2016 we were 4.7 million programmers. A year later we have grown to 6.6 million. (capture 2017-02-01)▼
April Fools, 2017. Internet security continues to be a joke. The gray button bar is absorbed into a shaken up menu bar. Stack Exchange is conspicuously missing from the top. (capture 2017-04-01)▼
So in February 2017 we were a community of 6.6 million programmers. By September, someone decided to loosen the definitions a bit and include visiting developers which tops us out at 50 million site users per month. This likely has something to do with courting business directly: "Stack Overflow Business Solutions: Looking to understand, engage, or hire developers?". Learn more why don't you!? DocumentationBeta has gone and Jobs -> Developer Jobs. (capture 2017-09-30)▼
We've captured the second annual Developer's Survey here and entered the modern era. (capture 2018-01-14)▼
Circa the beginning of this question, Stack Overflow Teams peeks it's head out with Slack integration. The top menu is de-cluttered by the addition of a side menu - I think this is configurable as mine is shown as a hamburger menu next to the Stack Overflow logo. (capture 2018-11-27)▼