Bochs if you don't need speed but lots of flexibility. You can use Bochs with GDB.

Qemu if you need more speed and less flexibility (it does dynamic translation so you loose a bit of speed) possibly a bit less safe than bochs. Its similar to Vmware and virtualbox actually derives from it. You can use GDB with Qemu.

Xen a friend of mine is quite confident in the Xen hypervisor as long as you set it up correctly which means using a serial loopback for configuration and other complex setup. However it would be very fast but still isolated and from what I understand it would garrantee your security as long as you controled the serial connection. Still Bochs is probably safest.

Bochs if you don't need speed but lots of flexibility. You can use Bochs with GDB.

Qemu if you need more speed and less flexibility (it does dynamic translation so you gain bit of speed but lose the acutal sequence of the instructions) possibly a bit less safe than bochs. Its similar to Vmware and virtualbox actually derives from it. You can use GDB with Qemu.

Xen a friend of mine is quite confident in the Xen hypervisor as long as you set it up correctly which means using a serial loopback for configuration and other complex setup. However it would be very fast but still isolated and from what I understand it would garrantee your security as long as you controled the serial connection. Still Bochs is probably safest.

Also while not an emulator/virtualization solution debuggers like SoftICE might be useful even though many softwares now detect and circumvent it.