Virtualbox is great for virtualizing Windows and Linux. That's all well and good, but they get boring after a while. The drop-down list in VBox has OS/2, Solaris, BSD, Netware, L4.. Yay.
There's also that.. "Other/Unknown" option. Hmm.. What "Other/Unknown" OSs have you tried in VBox?
(I don't use USB so I have it disabled and untested on all guests)
Install: A little tricky
Settings: 512MB RAM, 8MB Video, ACPI enabled, VT-x disabled, 1280x1024x16 custom video mode
Support: VESA video (w/3rd party app), network, sound (flaky)
Speed: Interface feels slow, disk access is pretty bad
Stability: Decent, but leave VT-x off!
Note: They still port Firefox and Seamonkey to BeOS but they run VERY slow
Install: Rough, but procedures can be found on Google
Settings: 512MB RAM, 8MB Video, ACPI/VT-x disabled, 1400x1050x32 custom video mode
Support: VESA video (after last patch), network
Speed: Probably beyond native, since the fastest PC it likely ever saw in it's day was barely a Pentium II
Stability: Good! (not that it can do much)
Note: virtualbox.org cannot properly render in nine year old web browsers.
Perhaps some Rhapsody?
Install: Same as OpenStep
Settings: 192MB RAM, 8MB Video, ACPI/VT-x disabled, video cannot go to VESA
Support: Nothing, really ; ;
Speed: Unbearably slow, due to the lack of VESA
Stability: Good, tho it did BSOD my host once, might have been VT-x on at the time.
Note: Rhapsody is one of the development platforms Apple released during the design and development of OS X. It is basically OpenStep with OS 9 stylings. It also marked an early interest Apple had in the Intel platform, as this is an X86 version, circa 1999.
Speaking of OS 9, let's cheat.
Windows 2000 guest running Mac OS 9.0.4 under SheepShaver, a PPC emulator. It runs as well as can be expected. SheepShaver is very glitchy and unstable, and crashes seemingly at random, but the speed in a VM is about the same as running it straight on XP64. I run it in a VM since SheepShaver can't do networking in XP64.