Like gasoline or carbohydrates, venture capital is the fuel that keeps the startup world humming.

2015 saw one of the most massive cash inflows to startups since the dot-com boom of the late 1990s. For better or worse, the money has pushed the tech world into bubble territory, and fostered legions of unicorn companies now valued at $1 billion or more.

Through the third quarter of 2015, venture capital firms plied Silicon Valley startups with $47 billion worth of investments, which included some of the largest funding events in history, some worth billions of dollars. It also saw the continued blossoming of known startup hubs, such as Los Angeles and New York City.

"2015 was one of the strongest years for venture capital investments ever," says Venky Ganesan, a managing director of Menlo Ventures. Also the incoming chairman of the National Venture Capital Association, Ganesan adds that 2015 ranks as the third strongest year for VC investment since the dot-com bubble, ranking after 1999 and 2000 respectively.

Though the tide may now be turning as investors take a more sober view of startup valuations, here's a look at five companies that made out like bandits in 2015:

1. Uber

The San Francisco-based ride-sharing company reeled in $2 billion from Foundation Capital and others in February and July. While it's facing regulatory challenges in its home city and internationally, the company is valued at a staggering $51 billion.

2. Airbnb

The apartment-sharing site, based in San Francisco, took in a whopping $1.5 billion in investment capital from CBC Capital, Fidelity Investments, General Atlantic, Kleiner Perkins Caufield & Byers, and others in June. It's valued currently at $25 billion.

3. Snapchat

The disappearing messaging app became instantly famous when it swatted away a $3 billion acquisition offer from Facebook in 2013. The company's founders knew what they were doing. In the second quarter of this year, the Venice, California-based company received $537 million from Benchmark Capital Management, Fidelity Ventures, General Catalyst Partners, Kleiner Perkins Caufield & Byers, and others. Although Snapchat's valuation has come under downward pressure in recent weeks from institutional investors, it is currently worth about $16 billion.

4. Zenefits

The cloud-based human resources provider received $500 million from Andreessen Horowitz, Fidelity Ventures, Khosla Ventures, and others in the second quarter. That round pushed its valuation to $4.5 billion. However, like Snapchat, the valuation of this San Francisco-based company has come under pressure as mutual fund company Fidelity marked down the value of its investment in Zenefits by nearly 50 percent in November. 

5. Palantir Technologies

The Palo Alto, California, security company crunches big data that allows it to detect patterns in seemingly unrelated events. It's been credited with helping to track down terrorist Osama bin Laden and helping uncover Bernie Madoff's Ponzi scheme, among other things. The company co-founded by Peter Thiel received $450 million in July, led by the Founders Fund, and is now valued at around $20 billion.

EXPLORE MORE Best in Business COMPANIESRectangle