With all the hoopla about Software-as-a-Service apps and mobile computing, it’s easy to forget that infrastructure software—everything from operating systems to middleware to databases—remains the foundation of every IT department and is critical for the operation of nearly every business. The worldwide market for operating system software alone grew 7.8 percent in 2010 to $30.4 billion, according to market researcher Gartner, while worldwide sales of application infrastructure and middleware software increased 7.3 percent to $17.6 billion.

Picking the top 25 infrastructure software vendors isn’t easy. We’ve focused on suppliers of the foundational technologies mentioned above and the tools for managing them. The list is dominated by such big names as IBM, Oracle and Microsoft. While it would be impossible for solution providers to partner with all—or even many of them—here’s the list of the industry players with whom they need to be familiar.