Avanade, the Seattle-based technology consulting company, this week released the results of its third global survey on cloud computing — finding that more and more companies are replacing on-site servers and internal tech operations with cloud-based services.

In fact, some of these services are becoming so popular that people inside companies are purchasing them without the knowledge of their IT staff.

Twenty percent of the respondents said they had purchased a cloud service without their IT staff knowing, despite the fact that 60 percent of companies surveyed have policies in place to prevent that from happening.

The top reason: It’s easier than getting the IT staff involved.

Avanade compares the trend to the influx of consumer technologies and social media in the workplace. From the company’s executive summary: “While nearly two-thirds of companies (60 percent) report they have corporate policies in place that prohibit such actions, respondents say there are no real deterrents for purchasing cloud services by stealth. In fact, 29 percent report there are no ramifications whatsoever and another 48 percent say it is little more than a warning.”

It’s one of several signs that cloud computing is becoming mature. The survey of 573 corporate executives in 18 countries found that 74 percent are using some form of cloud computing in their businesses, up 25 percent since a survey commissioned by Avanade in September 2009. The chart above, from Avanade, details the types of applications being shifted to the cloud, starting with email.

Here’s the executive summary of the study: PDF.

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