With all the power that today's computers have, why be content to run only one operating system on them? Thanks to modern advances in virtualization technology, you can run multiple operating systems on a single computer at near-native speeds. There are many choices for virtualization on Linux. On the proprietary end, you've got applications such as VMware, and on the open source end, you've got Xen, coLinux, and QEMU (for more information not covered in this chapter, see http://fabrice.bellard.free.fr/qemu/ , Mirror).
If you don't need a full-blown virtualization environment, the open source Wine suite may be just what you need: it's a call-level reimplementation of the Windows Application Programming Interfaces (APIs) that allows many Windows programs to run unmodified on Linux. With proprietary enhancements such as Cedega, Wine can even run many games!