There are various emulators for UNIX/RISC platforms available.
PersonalAlpha at the following site can apparently do Tru64, but I haven't tried it out. AFAIK it is from a DEC research project, so it should be fairly good.
http://www.emulatorsinternational.com/e ... lalpha.htm
BTW, you can also run DEC Ultrix using gxemul.
SimOS/MIPS can run a modified version of IRIX 5.3 on an IRIX/MIPS host, but is really slow when using Mipsy CPU model (embra just crashes, but I didn't look into it). SimOS/MIPS also runs on X86, but I couldnt try IRIX in it because the version I use in Linux/X86 has some custom hacks for some MIPS code I was testing. In theory it should work, but there might be some bugs to fix in SimOS first.
For SunOS/Solaris, there were some versions of SimICS that could emulate a SPARCstation. It might be only for SPARC hosts though.
I don't think there is an emulator for HPPA/HP-UX. HPPA isn't a nice architecture, and some of the more unusual features of it would be very slow to emulate.
Also, on the non-x86 front:
SimOS/PPC can do some modified versions of AIX, but requires a PPC host.
SimOS/Alpha can run a modified version of Digital Unix 4, but needs an Alpha host.