Here at ServerWatch we talk a lot about virtualization and the many different virtualization applications, and it is quite a mixed batch. Some are closed-source proprietary applications, some are open source, and some are a combination. Typically, a commercial product will also have a free-of-cost open source edition, which usually has a smaller feature set than the paid version. For folks who prefer an open source virtualizer that is the full-meal deal, give QEMU a try.
QEMU is versatile it functions as both a virtualizer and a machine emulator, although strictly speaking it is an emulator. Under full-system emulation it runs operating systems and applications written for different hardware platforms (e.g., running code for PowerPC on x86, or x86 on x86_64). You can create disk images and system snapshots, test all manner of networking functions between your guests (such as connecting VLANs, routing, tunneling, and file sharing), perform cross-platform development, and whatever else your imagination dreams up.
When you run x86-on-x86 code it functions as a virtualizer and executes guest code directly on the host's CPU with near-native performance, especially when you use the KQEMU accelerator. This used to be a closed source, free-of-cost application that users had to download and install manually. Now, it licensed under the GPL and available in the software repositories of most Linux distributions.
QEMU supports Linux, Mac OS X, and Windows, and you may be able to compile and run it on other operating systems. Visit QEMU for downloads and documentation.