What is history of this site? When and by whom was it created?
What were the important milestones of this site?
September 28, 2009: MathOverflow goes online
October 10, 2009: MathOverflow is announced at the Secret Blogging Seminar
June 2010: featured in the AMS Notices.
October 2010: 10000 users
June 2012: 100000th post (including the deleted ones)
June 24, 2013: MathOverflow joined SE network
September 17, 2014: Joel David Hamkins becomes the first user to reach 100k reputation.
February 11, 2019: 100000 questions
The historical first MO golden badge was awarded to Charles Siegel for this answer.
Update: Here is a historical snapshot (click for a better resolution) of top participators shortly after Charles earned his golden medal. This was shortly after Greg's sprint to (questionless) 10K reputations record. Note also that Andrew ranked 7th at the time has only bronze badges, and that Reid Barton has precisely 6666 points.
Now, on February 11, 2014 the count reached 100,000, and this in only slightly over 10 days, making this not only the currently most viewed question on MO, but the first question to reach 100,000 views in MO's history.
Fun side-aspect: the success of the question was so massive that some even expressed doubt this is happening for real.
In the unlikely event you have not yet seen it, here it is once again, the perfectly centered break according to Jim Belk.