Mark Cuban is one of the world's most successful entrepreneurs. The 61-year-old owns the Dallas Mavericks, stars on ABC's "Shark Tank" and is worth an estimated $4.1 billion.

His career, though, has had its fair share of lows. In his early 20s, Cuban was too broke to open a bank account and shared a $600-a-month three-bedroom apartment with five other guys. Cuban slept on either the floor or couch.