BinaryWolf

Virtual Machine Secure Browser

VMware Player is free software that enables PC users to easily run any virtual machine ( a self-contained operating environment that behaves as if it is a separate computer) on a Windows or Linux PC. The Player can run any previously configured virtual machine.

One is called the Browser Appliance and offers users a safe way of browsing the internet within a virtual machine using the Firefox web browser:

Protect Against Adware and Spyware: Users protect their PCs against adware, spyware and other malware while browsing the Internet with Firefox in a virtual machine. The Browser Appliance leverages virtual machine isolation capabilities to prevent malware downloaded in the browser from propagating to the normal desktop.
Safeguard Personal Information: The Browser Appliance can be configured to automatically reset itself after each use so personal information is never stored permanently.

They also offer a few versions of Linux virtual machines which can be run on Windows for people who are thinking of moving to Linux. The community-built virtual machines are, in essence, a complete, fully-set-up PC, emulated entirely in software, that runs inside your current operating system.

IPCop is a Linux firewall that is designed to be stable, secure, highly configurable, and easily maintained. Contains IPCop with several add-ons. 40MB zipped; 112MB unzipped.
m0n0wall is a popular open source firewall package, and makes a nice firewall or router for testing purposes in VMware. 7MB zipped; 8MB unzipped.
Sguil is the Analyst Console for Network Security Monitoring. Its main component is an intuitive GUI that provides realtime events from snort/barnyard. 88MB zipped; 280MB unzipped.