If you plan on running multiple operating systems on your Mac, one route you can take besides a direct installation like Windows in Boot Camp is to use a virtual machine, which installs the OS within OS X so it and its applications will run alongside your OS X applications.
There are several virtualization options for OS X, including VMware Fusion and Parallels Desktop, both of which offer robust solutions for running multiple operating systems that integrate the guest operating system well with the Mac OS. Recently, Parallels released an update to its latest version of Parallels Desktop that, in addition to a round of bug fixes, includes new options for installing and managing operating systems.
In Parallels Desktop 7, the new Wizard interface for setting up virtual machines has a Convenience Store feature for purchasing copies of Windows, in addition to direct links for downloading and installing other popular operating systems such as Ubuntu, Chrome, and Fedora, and even installing OS X Lion using its Recovery HD partition.
With the latest update, the Parallels Wizard now includes a quick way to access and install the latest Windows 8 developer preview in a virtual machine so you can test out Microsoft's latest OS. In addition, the update also provides a way to install OS X Lion directly from the Lion installation application that you download from the Mac App Store. While you could previously install Lion from the Mac App Store download, you first needed to open the installer package and access the InstallESD image file directly. Now you just need to select the installer application to install Lion.
While it may seem a bit odd to install Lion within Lion, in some instances it may be a useful thing to do, for example if you wish to test a software package before installing it in your main OS. Sequestering the software on a virtual machine will help you see how it installs and how it may run, and if a problem occurs you can easily remove the virtual machine and set it up again.
The update to Parallels Desktop 7 is available through the Parallels Desktop updater (access this from the Parallels menu within the program), but also can be downloaded from the Parallels Desktop Web site.