I believe that investors should always be looking for new places to put some cash to work. Over the last century, the S&P 500 has notched an annual gain more than two-thirds of the time. In other words, it's almost always a good time to put some money to work, historically speaking.

It's still important to invest money in only the most promising companies, even if we are talking about a relatively small amount, such as $1,000. Adtech company PubMatic (PUBM 1.75%), restaurant-technology company Toast (TOST 3.42%), and online learning marketplace Udemy (UDMY 2.73%) are three such promising companies, and I believe each could double an investment for patient investors.

Here's a look at each one.

1. PubMatic

PubMatic is an adtech company that helps publishers sell their ad slots. The company doesn't only benefit from growth in digital advertising, although that helps. It also benefits by winning more business from its existing publisher customer base.

Publishers use many adtech companies simultaneously. But with its supply-path optimization, PubMatic helps its customers whittle it down to just a couple of partners. This helps the company win more business.

It's clearly working. In the third quarter of 2023, 45% of PubMatic's activity was from supply-path optimization deals, which was an all-time high. Moreover, it processed 56 trillion ad impressions during the quarter, which was also a record.

Ad rates are down right now, so PubMatic's revenue growth has stalled and its gross margin has slipped. However, the advertising market is expected to pick back up.

Consequently, I expect the company's revenue growth and its gross profit to rebound soon. And it has invested in its infrastructure in recent quarters, which can provide an additional gross-profit boost.

With $171 million in cash and marketable securities for PubMatic as well as no debt, there's no reason to doubt the company's financial fortitude. It's unclear how long the slowdown in the ad market will last.

But PubMatic can outlast this headwind and is positioned for profit growth when conditions improve. This is why it's a prime candidate to double an investment for those who buy shares today.

2. Toast

This past summer, Toast introduced a fee on transactions that was poorly received. It quickly got rid of it because of the backlash.

The jury is still out on whether it suffered permanent loss of goodwill with its customers -- only time will tell. But assuming it moves past this issue, Toast stock should be a great long-term buy.

With nearly 100,000 restaurant locations using its technology, Toast is undoubtedly enjoying robust adoption. The company allows restaurants to process orders, manage delivery, handle kitchen displays, invoice customers, and more. And most of its customers currently only use a fraction of its available modules, providing revenue upside as the restaurants elect to use more over time.

Toast appears to benefit from word-of-mouth marketing. According to management, the more of a local market that it wins, the more interest automatically comes in from other restaurants.

With new customers coming aboard, and old customers adopting more services, I expect Toast to enjoy strong long-term revenue growth. And it's increasing from a large revenue base. As of the third quarter of 2023, annualized recurring revenue was $1.2 billion, up 40% year over year.

With a market capitalization of $9.3 billion, Toast looks like a good bargain for investors today in light of its high growth in recurring revenue.

3. Udemy

Udemy is a user-generated course platform for people looking to learn without a degree. With over 75,000 instructors and over 210,000 courses in about 75 languages, the company's catalog is extensive.

It started as a platform for individuals to learn. But management wised up and realized it could package its content as a subscription for businesses to upskill their workforces.

Udemy's enterprise business now accounts for more than 50% of its revenue. And its annualized recurring revenue for this subscription product is now $443 million and growing 26% year over year. This outpaces growth in its consumer segment and shows the direction the company is moving in.

Even with the fast growth of Udemy's enterprise service, only a little more than half of Fortune 500 companies currently subscribe. So there could be plenty of growth left. Indeed, management believes it's pursuing a $4 billion opportunity with its enterprise business, which is nine times larger than its current recurring-revenue base.

Udemy had been paying its instructors 25% of its subscription revenue. But on Jan. 1, it dropped this down to 20%, and it plans to drop it down to 15% by 2026. This could anger instructors and motivate them to leave -- a potential problem worth monitoring. However, if they stay on Udemy's platform, this will boost the company's profitability considerably.

Companies that can grow profits tend to have good long-term stock appreciation. That's why I believe Udemy shares could double investors' money if the business continues on its current trajectory.

As far as technology stocks go, PubMatic, Toast, and Udemy are a little more obscure than most. But all three are on strong financial footing, have a large opportunity, and have a realistic path forward. That's why all three are worth investors' consideration today.