System Emulators / Virtual Machines

Emulators for x86

QEMU (homepage) is a fast, robust program that emulates a bunch of different processors (including x86 and PowerPC) and runs on a bunch of different processors. I've got local mirrors for:

QEMU on Windows. Unzip to C:\Program Files\qemu\. (port is from cheblossom; updated 2006/04 to include -net option)
QEMU on Mac OS X (port is called Q)
QEMU on Linux (binaries I built--install with "cd /; tar xzvf ~/qemu-0.8.0-linux.tar.gz")
QEMU source code version 0.8.0

Boot from any disk image with: "qemu disk.vmdk" (Add "-L C:/Progra~1/Qemu" on Windows so QEMU can find its BIOS images.)
Build a new blank 2GB disk image with: "qemu-img create -f vmdk disk.vmdk 2G".
Boot from an install or live CD: "qemu disk.vmdk -cdrom /dev/cdrom -boot d".

VMWare Player emulates an x86 machine on another x86 machine. The commercial version of VMWare is expensive, but the player is free (player allows you to run virtual machines, but not easily create or manipulate them).
Bochs is an older x86-only emulator. It's not nearly as good as QEMU or VMware player.
Xen is an x86-only virtual machine that requires a modified guest operating system (at least until x86 virtualization hardware improves).

The "-net user" command-line option to QEMU will emulate a 'user-mode' network (gateway 10.0.2.2, DNS 10.0.2.3, with DHCP), forwarding traffic into and out of the virtual machine. This means you can download code from web and FTP sites from inside the virtual machine. Note that this works in addition to other -net commands that emulate a virtual network.
The "mkisofs" command (part of cdrtools for linux or Windows) can create a .iso CD image for importing data into a virtual machine. Build the image like "mkisofs -J -R -o out.iso my_stuff", and then run "qemu -cdrom out.iso ...".
The 30-day evaluation version of WinImage can mount, read, and write .iso or .img RAW disk images.
On a UNIX or Linux box, the root command "losetup -o 32256 /dev/loop0 disk.img; mkdir -p tmp; mount /dev/loop0 tmp" will mount the first partition of a RAW disk image for read or write. You can convert your .vmdk image to RAW format with "qemu-img convert disk.vmdk -O raw disk.img".

For UAF students, I have a collection of virtual machine images I prepared for CS 493/693. The collection is available on DVD (stop by my office) or for download (only works from the uaf.edu domain). The Windows images can only legally be run on UAF-owned hardware.
I've collected a few images hosted locally in the x86 directory:

ReactOS (project) is a free clone of a Windows-like operating system. (10MB download; 50MB unpacked)

H2K is my own small distribution of Linux, including compilers and network servers. (26MB download; 90MB unpacked)

Bochs hosts a bunch of really tiny bootable installed disk images. Some of these are broken, but others work fine.
VMWare hosts a number of community VM images for Linux and other free OSs. Some of these are VMWare-specific, and won't work in QEMU.
FreeOSZoo has fairly large images of every flavor of Linux, FreeBSD, NetBSD, OpenBSD, FreeDOS, and other OSs.

PearPC emulates the PowerPC processor. It runs under x86 on Windows and Linux.
minivmac emulates the Mac Plus (Motorola 68000 processor). BasiliskII emulates the Mac II/Quadra line (68020, 68030, or 68040). It's not legal to run these emulators unless you own ancient Mac hardware.
SPIM is an emulator for the (now obsolete) SGI MIPS processor. I used this in Dr. Roth's CS 301 back in 1996!