I use Virtual Box, but it is based on QEMU. I used QEMU sometimes because it has support for MIPS and SPARC emulation so that it can run old operating systems

I use Virtual Box, but it is based on QEMU. I used QEMU sometimes because it has support for MIPS and SPARC emulation so that it can run old operating systems for those platforms on it. I had a task to modify QEMU to get networking working on a Solaris 7 SPARC virtual machine, but there is a bug in the kernel that causes a crash with the NIC that is emulated in QEMU for SPARC and there used to be a Sun Y2K CD-ROM that fixed it and nobody seems to have it anymore. Anyway I couldn't modify QEMU in time to make it work and lost that gig. To be honest there are Russian Hackers trying to get Solaris 7 and older stuff working in QEMU and can't seem to get anywhere. But that was a few years ago and maybe someone made progress yet? QEMU runs IBM OS/2 quite well.