Boot Camp (Free) - Comes with all Intel Macs
It allows multiple operating systems to run on your computer. Virtualbox is not quite as lightweight as Q. It supports emulating x86, x86-64bit, PPC PowerMac, PPC PREP, SPARC32, MIPS, and ARM processors. Oddly enough, it runs on PPC Macs, in addition to Intel Macs. Q is a great lightweight CPU emulator for OS X. Professionally supported enhancement of Wine Q (Free) Wine (and Crossover) do not require an actual copy of Windows installed on your machine. I would recommend Winebottler, a free binary installer that also allows you to turn a Windows executable into a simple application that launches in X11. You do need to install the Developer Tools, included on your computer's install disc. I would also not recommend compiling it yourself. I don't know how well it works with 3D stuff.
I highly recommend Wine, which allows for running Windows applications without actually emulating the whole Windows OS. When using the term 'emulator', it is important to distinguish between something that acts like Windows to applications (like Wine/Crossover) and something that acts like a PC to Windows (most of the other solutions) so that Windows can be installed.
