The tech sector is known for generating millionaire-making gains for patient investors. However, the world's top tech stocks -- like Nvidia, Microsoft, and Apple -- are already worth trillions of dollars and probably have less upside potential than the sector's smaller and lesser-known stocks.

So today, I'll focus on three underappreciated small- and mid-cap stocks that could potentially deliver millionaire-making gains over the next few decades: Reddit (RDDT -0.05%), Fiverr International (FVRR 1.33%), and DigitalOcean (DOCN 1.74%).

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1. Reddit

Reddit's stock price has risen nearly 70% since its public debut in March. The social media company impressed investors with its robust growth rates and new data-sharing artificial intelligence (AI) deals with OpenAI and Alphabet's Google. In 2023, Reddit's revenue rose 21% as it narrowed its adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) loss. In 2024, analysts expect its revenue to grow another 36% as its adjusted EBITDA turns positive.

Reddit's growth is accelerating as it gains more users and the macro headwinds for the advertising market dissipate. In the first quarter of 2024, its revenue rose 48% year over year, its daily active unique users grew 37% to 82.7 million, and it generated a positive adjusted EBITDA. Reddit carved out a defensible niche in the crowded social media market with its user-directed discussion forums, and certain major events -- including the COVID-19 pandemic, the WallStreetBets meme stock phenomenon, the Russo-Ukrainian War, the Israel-Hamas War, and various U.S. elections -- drove more users to its platform.

Analysts expect Reddit's revenue and adjusted EBITDA to rise 22% and 91%, respectively, in 2025. Based on those estimates, Reddit's stock still looks reasonably valued at 4 times next year's sales and 22 times its adjusted EBITDA. If it successfully scales up its business and widens its moat over the next few years, it could generate millionaire-making gains.

2. Fiverr International

Fiverr went public at $21 a share five years ago, and its stock hit an all-time high of $323.10 in February 2021 before dropping all the way back to about $22. The gig economy marketplace initially grew rapidly throughout the pandemic, but it lost its momentum as the macro headwinds drove companies to hire fewer freelancers. Some investors were also concerned that new generative AI apps could displace a lot of its freelance positions.

Fiverr's revenue only rose 7% in 2023, compared to its 13% growth in 2022 and 57% growth in 2023. But on the bright side, its adjusted EBITDA more than doubled to $59 million as its take rates increased throughout the entire year. Those improvements suggest it's merely experiencing a cyclical slowdown in a challenging macroeconomic environment instead of facing existential threats from new AI algorithms. It insists the growth of the AI market will actually generate long-term tailwinds for its business as more companies roll out freelance positions for AI-related fields.

In 2024, analysts expect Fiverr's revenue and adjusted EBITDA to rise 6% and 20%, respectively. But in 2025, they expect its revenue and adjusted EBITDA to grow 11% and 24%, respectively, as the macro environment stabilizes. Those are solid growth rates for a stock that trades at just 2 times this year's sales. If the gig economy warms up, Fiverr's growth could reaccelerate as its stock is revalued as a growth play again. It could eventually generate multibagger gains as that happens.

3. DigitalOcean

DigitalOcean is a cloud infrastructure company that carves out smaller "droplets" of servers for smaller customers at lower prices. It also added GPU-powered AI processing capabilities to its servers by acquiring the start-up Paperspace last year. Those services make it an attractive alternative to bigger cloud platforms, which mainly serve large enterprise customers.

DigitalOcean went public in March 2021 at $47 per share, and its stock nearly tripled to an all-time high of $130.26 in November 2021 before pulling back to $34. Like many other cloud companies, its growth decelerated as the macro headwinds drove many companies to rein in their spending. Its revenue only rose 20% in 2023, compared to its 34% growth in 2022 and 35% growth in 2021. Analysts expect its revenue and adjusted EBITDA to only grow 11% and 5%, respectively, in 2024.

That deceleration was disappointing, but it's still gaining new customers and its adjusted EBITDA margin is expanding. Therefore, its growth could quickly stabilize again as the macro environment improves. For 2025, analysts expect its revenue and adjusted EBITDA to rise 13% and 14%, respectively, as the near-term headwinds dissipate.

Those are solid growth rates for a stock that trades at 4 times this year's sales at 14 times its adjusted EBITDA. If its growth accelerates again and its valuations rise, it might generate millionaire-making gains over the next few decades.