The New York Stock Exchange (NYSE) and many other financial institutions run their most critical services and applications on Linux. The NYSE specifically uses RedHat Enterprise Linux because of its stability and support.
Linux is the most-chosen operating system in the U.K, France, Germany, and other European countries due to its open source programming. Major companies like LVM Insurance and Deutsche Bank rely on Linux's time-tested stability, transparency, high uptimes, and low maintenance. LVM Insurance in Muenster, Germany converted all 7,700 of its Desktop computers to Linux.
Reuters Group, PLC. converted their Reuters Market Data System to Linux stating that more than half of their 20 largest customers had converted to Linux.
The two big reasons why companies shift their critical computing or desktops to Linux: Price and Performance.
Linux wins bigger in the Data Center than on the Desktop, even in Europe, but there are many companies worldwide now considering conversion to Linux due to the rising costs of Windows software, the need to upgrade for each new Windows version, and performance concerns.
In response to the loss of Data Center space, Microsoft's Server 2008 can be installed as a Core Server without the standard top-heavy graphical interface. Will it help Microsoft's diminishing significance in the Data Center? Maybe, maybe not.