One of the mega growth companies of the past few years is Salesforce.com (NYSE : CRM). Since 2008, sales have spiked from $748.7 million to $1.3 billion. And investors have piled into the stock, which has gone from $34 to nearly $120 since early 2009.

Salesforce.com builds software to help companies improve the results of their sales teams. While this is not new, the company has taken different approach to delivering its solutions through something known as “cloud-computing.” And while much of the gains to be had in this stock have already been made, the good news is that there are other stocks in the sector that are also growing at a rapid clip and should provide nice returns for investors.

To understand cloud-computing, it is important to take a look at the traditional approach to business software. Known as on-premise software, this involves installing complex applications on a company’s servers. This means there are large expenses for information technology (IT) systems as well as technical support staff and outside consultants.

On-premise software is far from cheap. A company must pay an upfront licensing fee and then ongoing maintenance fees. So if a company implements an enterprise resource planning (ERP) system — which handles things like the general ledger, inventory, payroll, and so on — the costs can easily amount to several million dollars.

But cloud computing takes a much different approach. First of all, the software is typically installed on the software provider’s own servers. The customer simply accesses the software via the Internet. The result is that it is much easier to update the application and there is also no need to invest huge amounts of money on an IT infrastructure.

Even the business model is different. For example, cloud-computing providers charge an ongoing subscription fee, which is usually based on the number of users. For the most part, this is cheaper than the licensing/maintenance approach.

All in all, cloud-computing looks like a disruptive technology, and the market opportunity is enormous. Consider that the International Data Corporation (IDC) pegs the cloud computing market at $16 billion this year and forecasts it to reach $56 billion by 2014.

So what are some top cloud-computing operators that are attractive investment opportunities? Here’s a look at three:

1. SuccessFactors (Nasdaq: SFSF) develops business execution software. That is, it helps communicate key strategies throughout an organization and measure the ongoing results.

#-ad_banner-#Since 2008, revenue has grown about +27% each year. For the current year, SuccessFactors is expected to post revenue of $198 million to $200 million and has actually raised guidance twice. More than half of new revenue comes from existing customers, which shows that the software is providing a strong value proposition. SuccessFactors also continues to invest heavily in its technology and has purchased several companies. The company estimates its market opportunity at a whopping $36 billion. It helps that the company’s software can scale from small businesses to global enterprises.

2. NetSuite (NYSE: N) develops an ERP system for the cloud. While the market is dominated by large companies like SAP (NYSE: SAP), Oracle (Nasdaq: ORCL) and Microsoft (Nasdaq: MSFT), they have been slow to move to the cloud.

No doubt, this has been a boon for NetSuite. In the latest quarter, revenue increased +17% to $47.1 million, which was above the $45.3 million guidance. A driver for the company is its OneWorld system. This is focused on the needs of global customers who are willing to pay premium prices for a strong ERP system.

As a sign of the growth prospects, NetSuite said it will be boosting expenditures on its platform and also increasing the ranks of its sales team.

3. Taleo (Nasdaq: TLEO) develops a cloud-computing offering for talent management applications, helping with things like recruiting, management and tracking of employees.

Even with a prolonged high rate of unemployment, Taleo continues to grow its business. In the latest quarter, revenue increased +14.6%. As the economy comes back, expect the growth rate to ramp up even more.

Taleo has also made several smart acquisitions to bolster its offerings. For example, the company recently shelled out $125 million for Learn.com, which is a top provider of learning management tools for employers. With the deal, Taleo was able to pick-up more than 500 customers.

Action to Take –> While these three cloud operators are top performers, I would give priority to NetSuite as an investment. The company has a comprehensive solution, which has taken more than 10 years to build, and the barriers to entry are significant. As NetSuite goes up-market in terms of targeting customers, there should be a nice boost in revenue, which should drive significant returns for investors.