QEMU

Qemu is a x86 Emulator/Virtual Machine.

http://fabrice.bellard.free.fr/qemu/

Grabbing debug messages:

By default debug messages from ReactOS are sent to the serial port (COM1).
For grabbing that output, you need at least qemu version 0.6.1 where you can specify the -serial option.
Like this:
 $ qemu -serial stdio -cdrom roslive.iso -boot d

If you are testing the setup cd, note that you can change the kernel arguments 

by editing the txtsetup.sif file (so things actually get written to the serial port).