Mt. Gox
Mt. Gox was a Bitcoin company in the Exchanges category, founded in 2010 by Jed McCaleb (original), Mark Karpeles (acquired 2011).
The Idea
Allow for easier access to buying and selling Bitcoin. McCaleb repurposed a domain he'd bought in 2007 for a Magic: The Gathering card trading site ('Magic: The Gathering Online eXchange') and launched a Bitcoin exchange on it in July 2010. He sold the site to French developer Mark Karpeles in March 2011.
Early Traction
Allowed anyone to deposit USD and buy Bitcoin, opening up access to those outside of miners.
Peak Stats
Mt. Gox handled 70-80% of all Bitcoin transactions, and held ~850,000 Bitcoin on behalf of users.
Status Today
Collapsed in February 2014 after revealing that ~744,408 customer BTC and ~100,000 of its own BTC had been stolen through a hack that went undetected for years due to poor security and controls.
Why It Matters
Mt. Gox's collapse became the defining cautionary tale of the industry and catalyzed the push for both self custody to reduce counterparty exposure, as well as regulated, institutional-grade custody and exchange infrastructure. Jed McCaleb went on to co-found Ripple and later Stellar.