2021 has been good to investors, with the stock market steadily climbing higher. But September is historically a tough month. The "September Effect," while basically just a superstition, struck in full force last year when the market dropped nearly 10% in the first three weeks of the month.

If the September Effect hits us again this year, our contributors have five stocks to suggest that you could pick up at a discount: Insulet (PODD 0.30%), Amazon (AMZN 0.83%), Shopify (SHOP 0.08%), Cresco Labs (CRLBF 2.72%), and PubMatic (PUBM -2.41%).

1. Insulet: Pumping up profits

Todd Campbell: Medtech innovation is improving millions of lives, and perhaps some of the biggest advances are occurring in diabetes treatment. Historically, managing type 1 or type 2 diabetes involves finger sticks to determine blood sugar levels, and often, multiple daily injections of insulin to bring patients back into their desirable ranges. Today, pumps and sensors are performing those tasks; and increasingly, automated systems are eliminating finger sticks and injections altogether.

Person writing something down while looking at laptop.

Image source: Getty Images.

Medtronic and Tandem Diabetes were the first companies to offer next-generation, automated insulin delivery systems, but stiff competition from Insulet's Omnipod 5 could be coming soon. The Food and Drug Administration is expected to make a go/no-go decision on Omnipod 5 in the fourth quarter, and if it's approved, it will be the first tubeless, water-resistant, automated system available.

There's reason to think Omnipod 5 will be a hit. An absence of tubing means less risk of accidentally dislodging the pump, and a water resistance feature eliminates the need to remove the pump while showering or swimming.

Also, insulin pumps and continuous glucose monitoring (CGM) systems have traditionally been treated by insurers as durable medical equipment, making them prohibitively expensive. Insulet's system will be available at pharmacies, making its cost more comparable to the monthly cost of daily injections. According to Insulet, the average person buying its pumps at a pharmacy today pays less than $50 per month out of pocket.

Only about one-third of the 1-million-plus type 1 diabetics in the United States currently use a pump, and even fewer of the 30 million type 2 patients are pump users, so the opportunity for revenue and profit growth is significant if Insulet's Omnipod 5 gets a regulatory green light.

2. Amazon: A Prime opportunity

Adam Levy: Amazon share prices took a tumble after the company reported its second-quarter earnings results, and that's when I took the opportunity to buy more of the mega-cap stock. While investors were disappointed with the muted top-line growth for Amazon and management's third-quarter outlook, I thought there was still a lot to like about the earnings results.

Online store sales grew just 13% year over year last quarter, but that's still better than Amazon's biggest rivals in online retail like Walmart (6%) and Target (10%). Moreover, third-party seller services continue to hold up well, up 34%, driving a more profitable online retail operation. Meanwhile, Amazon's other big businesses -- cloud computing and advertising -- saw revenue growth accelerate. And those are the real profit drivers at Amazon.

Indeed, operating profits climbed to $7.7 billion, up from $5.8 billion in the second quarter last year. Management's third-quarter outlook calls for a decline in operating profits, though. That's due to ramping up its investment in fulfillment capacity, filling the massive 50% footprint expansion it built last year with equipment, inventory, and workers. Operating profits continue to expand long term, fueled by AWS, advertising, and third-party services like Fulfilled by Amazon. All three of those businesses look exceptionally healthy.

With shares trading around the same level as this time last year, there's still an opportunity to buy Amazon stock.

3. Shopify: E-commerce is nowhere near finished growing

Chris Neiger: The pandemic helped convince some e-commerce holdouts to finally jump on the online shopping train, but there's evidence proving that this market still hasn't reached its peak. For example, in the second quarter of 2021, just 12.5% of all U.S. retail sales were made online. 

The rest of this untapped market is where Shopify's future opportunities exist. The company has already done a phenomenal job attracting businesses of all sizes to use its e-commerce building platform. To date, the company has more than 1.7 million businesses using its services, and the company's most recent quarter (reported on July 28) shows that the company is still growing like a weed

Sales in the second quarter increased by 57% -- topping $1 billion for the first time -- and monthly recurring revenue popped by 67% to $95 million. The company's two main revenue segments -- subscription solutions and merchant solutions -- grew quickly, rising 70% and 52%, respectively. 

Some investors may look at the company's 955% share price gains over the past three years and question whether this stock has any room left to run. But when you consider that just a sliver of all retail sales occurs online right now, you begin to see the long-term opportunity for Shopify. 

As more companies begin building out online shops in the coming years, many of them will turn to Shopify's industry-leading platform. That should help fuel more growth for the company and potentially provide investors with sizable returns as the e-commerce market continues to gain more market share.

4. Cresco Labs: Ready to soar

Keith Speights: Less than six weeks into 2021, Cresco Labs stock was up more than 70% year to date. Since then, though, nearly all of those gains have evaporated. But Cresco's prospects remain as compelling as ever.

The company ranks as one of the leading U.S. multi-state cannabis operators. It's the No. 1 cannabis wholesaler in the country. Cresco operates in 10 states, including six of the top 10 biggest markets in the U.S. 

While many cannabis operators dream of achieving profitability, Cresco posted positive earnings in its latest quarter. Its sales soared nearly 123% year over year and jumped almost 18% above the previous quarter. 

I expect continued momentum. The cannabis markets in several of the states where Cresco currently operates are only in their early innings. The company is opening new retail cannabis stores in key markets. It's also making additional acquisitions, most recently announcing a deal to buy a Maryland medical cannabis dispensary.

And Cresco stock is actually quite cheap. Shares trade at only 3.3 times trailing-12-month sales. The company projects a revenue run rate of $1 billion by the end of 2021, which makes the pot stock's valuation even more attractive on a forward-looking basis.

There are also some big wild cards that could provide catalysts for the stock. For example, more states could legalize cannabis. It's possible that federal cannabis reform could be enacted. 

Analysts think that Cresco's shares could skyrocket nearly 200% from their current levels. My view is that they're probably right.

5. PubMatic: Advertising can be exciting (if you own the right stock!)

Sean Williams: Although we're entering a stretch that's historically not been great for investors, programmatic ad-tech stock PubMatic is simply too intriguing a value to pass up.

PubMatic is what's known as a sell-side platform. This means it uses its cloud-based infrastructure and machine-learning algorithms to work with publishers to optimize ads and sell their display space. Though publishers maintain control over some aspects of the process, such as the minimum accepted price to sell their display space, it's designed to be an automated process.

What's particularly exciting for PubMatic is that cord-cutting is pushing content right into its wheelhouse. Through at least 2025, the company foresees consistent double-digit growth potential in mobile, video, and connected TV (CTV)/over-the-top (OTT) programmatic ads. CTV/OTT is easily its biggest long-term growth opportunity.

Make no mistake about it, publishers are on board with the value proposition offered by PubMatic's platform. Sales have consistently come in ahead of Wall Street's and the company's own expectations, and existing publishers spent 50% more in the June-ended quarter than they did in the year-ago period. Altogether, rapid growth in CTV/OTT has helped push PubMatic's growth rate to roughly double the digital ad market.

The bottom line is that PubMatic is already profitable on a recurring basis and can be scooped up for about 6 times sales in 2021. In an industry where double-digit sales multiples are common, PubMatic stands out as a serious value.