No. 5: Snapchat
Valuation: $16-20 billion
CEO: Evan Spiegel
Founded: 2012
What it does: Its photo-messaging app allows users to send photos and videos that automatically disappear after a set period of time.
Total funding: $1.2 billion
Notable investors: Yahoo, Kleiner Perkins, Benchmark Capital, Lightspeed Venture Partners, Coatue Management, SV Angel
No. 4: Palantir
Valuation: $15 billion, but $500 million in new funding could bump up its valuation to $20 billion
CEO: Alexander Karp
Founded: 2004
What it does: Palantir is a software and services company that specializes in data analysis. Some of its biggest clients are government agencies like the CIA and the FBI.
Total funding: ~$1.5 billion
Notable investors: Founders Fund, Tiger Global Management, Glynn Capital Management, Jeremy Stoppelman
No. 3: Airbnb
Valuation: $24 billion
CEO: Brian Chesky
Founded: 2008
What it does: Through its app and website, Airbnb provides a marketplace for people to rent out rooms or their entire homes to other people.
Total funding: ~$1.8 billion, pending a fresh $1 billion round of funding
Notable investors: SherpaCapital, T. Rowe Price, Founders Fund, CrunchFund, Sequoia Capital, Andreessen Horowitz, DST Global, General Catalyst Partners, SV Angel, Greylock Partners
No. 2: Uber
Valuation: $41.2 billion, though a new round of funding could raise its valuation to $50 billion.
CEO: Travis Kalanick
Founded: 2009
What it does: Uber's taxi-hailing app connects its users with drivers of private vehicles under contract with Uber. It offers different types of cars, from full-size luxury cars to smaller vehicles.
Total funding: $5.9 billion
Notable investors: Benchmark Capital, Menlo Ventures, Google Ventures, KPCB
No. 1: Xiaomi
Valuation: $46 billion
CEO: Jun Lei
Founded: 2010
What it does: Xiaomi is a Chinese electronics brands that manufactures smartphones. The company's smartphones are considered among the best that run Android, and the company has hardcore loyalists as Apple does in the US.
Total funding: $1.4 billion
Notable investors: Digital Sky Technologies, Hopu Investment Management Company, DST Global, IDG Capital Partners, Qualcomm Ventures, Morningside Group
BONUS: These companies aren't quite $20 billion companies, but they could be soon.
SpaceX, Elon Musk's space exploration company, designs, builds, and launches vehicles for space exploration. SpaceX has raised $1.2 billion at a $12 billion valuation.
Flipkart is an India-based e-commerce site specializing in electronics and content like books and music. It has a $15 billion valuation and has raised $2.5 billion in funding.
Virtual pinboard Pinterest has raised $1.3 billion at an $11 billion valuation.