Infrastructure as a Service
Your typical cloud standard for a platform—or for that matter, most of the Internet—consists of a solution stack known as LAMP, which stands for Linux, Apache, MySQL, and one of a few scripting languages, including Perl, PHP, and Python.
Linux is an incredibly powerful OS for this purpose, really the OS of preference when it comes to cloud computing infrastructure. This includes raw servers and storage.
Amazon, for instance, actually runs a cloud computing-style service called Elastic Compute Cloud, or EC2 where one can pay for whatever amount of server, storage and network capacity you need and also provides you with many in-browser tools for managing all this. A number of Linux distros are accommodated with EC2, including Ubuntu, Red Hat, Debian OpenSolaris, Fedora, openSUSE, and more—all Linux options being significantly cheaper (and significantly more popular) than their Windows counterparts.
Some other examples include Eucalyptus Project, which provides for an open source infrastructure for cloud computing, and Zimory, which caters exclusively to Linux operating systems.