Financial technology, or fintech, has been around as long as credit cards. The granddaddy of fintech is probably Visa, which started the war on cash 61 years ago when Bank of America invented the credit card. Today Visa is a mega-cap worth over $400 billion. And it's still growing.

Other fintech stalwarts include payment companies PayPal and Square. PayPal is already a huge company, with a market cap of $124 billion. The stock has tripled since it went public three years ago. Square has done even better -- its market cap has increased by five-fold over the same time span.

And while these stocks are great investments, it's the small caps in fintech that might have the greatest possibilities for new investors in this space. Here are three very interesting fintech stocks that are worth the time to consider for your portfolio: Afterpay (OTC:AFTPF)StoneCo (NASDAQ:STNE) and Zillow (NASDAQ:Z)

1. Afterpay: The war on credit?

Australia's Afterpay offers a revolutionary way to shop. Using its payment system, consumers receive the equivalent of an interest-free loan on purchases made at participating retailers. You initially make a 25% payment of the purchase price of your item, and Afterpay pays the remaining 75%. Then you reimburse Afterpay with three installments over the next three fortnights. Six weeks after your initial purchase, you are free and clear.

happy shopper

Image source: Getty Images

Consumers love the arrangement since it's an interest-free loan, and it allows them to buy expensively priced things without paying all the money upfront. Afterpay makes its money by charging the retailer 4% for its service. And participating retailers are willing to pay the fees because Afterpay drives customers into the retailers' shops. 

Millennials, in particular, are enthusiastic users of Afterpay. The company does no credit checks, so it's perfect for shoppers with bad credit or no credit. And Afterpay says user default rates are very low (0.6%). Afterpay just gave a financial update of its business results over the last four months. The company's growth is still fantastic.

  July to October Growth  YOY
Underlying sales AU$2.7 billion (US$1.84 billion) 110%
Active customers 6.1 million 137%
Active merchants 39,450 96%

Data source: Afterpay. YOY = year over year.

Afterpay is adding 15,000 new customers every day. And the company's list of participating retailers is getting more and more impressive. Recently, it's added eBay (Australia), Ulta Beauty (U.S.), and Marks & Spencer (U.K.) to its roster. The company estimates that its underlying sales will hit AU$20 billion by 2022. Analysts for Goldman Sachs think this number is too conservative and believe Afterpay will have AU$29.2 billion in underlying sales in 2022. 

2. StoneCo: Berkshire Hathaway's growth stock

StoneCo's claim to fame in the U.S. is that Berkshire Hathaway (NYSE:BRK-A)(NYSE:BRK-B) owns 5% of it. StoneCo is a payments company similar to Square, and it's dominating in South America. The stock is very pricey at 192 times trailing earnings. But this optimism might be justified because the company has amazing numbers, including profit margins of 60%, revenue growth of 57%, and earnings growth of 180%.  

The competition has been spiking for the company. Back in April, Brazil's Itau Unibanco (NYSE:ITUB) announced that it would start advancing cash to merchants without charging any interest. StoneCo's stock immediately dropped 23% on the news. 

Nonetheless, it's been a stellar year for the stock. Shares are up 89% since the beginning of the year. (Itau Unibanco is down 14% over the same time frame). One of the big drivers for StoneCo is that it offers merchants a software solution, not just advantageous financial terms for payments.

In its most recent quarter, the company had 360,000 merchants actively using its services overall. The software side of the business is growing geometrically. StoneCo had 32,000 subscribers to its software solution in January; that number had more than doubled to 70,000 subscribers by July.  

So far, the company has not had to reduce its interest rates on its payments business; it's holding steady at 1.85%.

3. Zillow: A new way to buy a house

Zillow is a battleground stock for bulls and bears. One of the stars of The Big Short, Steve Eisman, is publicly short the stock. He calls it a "terrible business" and is dismissive of the idea that technology can change the real estate market.

Meanwhile, the co-founder of Zillow, Rich Barton, has had fantastic success using the internet to revolutionize how you go on vacation, how you buy or sell a house, and how you look for a job. While he was at Microsoft, Barton started up the website that would become Expedia. Then he co-founded Zillow and followed that by co-founding Glassdoor.  

Barton is an expert at using information technology to find and create value. Zillow, Glassdoor, and Expedia are all similar businesses that empower consumers by giving them information in the various markets to see what is available.

The real estate market has been rocked by the rise of iBuyers. These are companies that are trying to simplify the housing market by making it faster and easier to buy and sell a house. Zillow has been long-established as the king of information when it comes to house valuations. People have been going to Zillow for years to get a free estimate (a "Zestimate") on how the market is valuing a property. Now Zillow is putting its money where its mouth is and is making real offers to buy certain properties in certain markets.

Eisman and other shorts (26% of its shares are sold short) dispute the validity of Zillow's information. Eisman believes that real estate is a local market and that Zillow is ignorant of the market worth of the houses it is buying. Zillow is confident that its data gives it an advantage in the market. And the company only buys houses in limited situations. Maybe 2% of people who request an offer end up selling their home to Zillow. The other 98% are valuable leads the company can market to real estate agents. Zillow's ultimate goal is not to flip houses for profit, but to morph into a one-stop-shop where the real estate market will operate quickly and efficiently. If Zillow pulls this off, the sky's the limit.