Installing 20+ operating systems on Mac without any virtualization
Depending on your Mac Pro the following OSs should run (or not). I didn't include every Mac Pro ever sold, but I tried to list major development steps ( e.g different EFI-architectures 32bit->64 bit):

                     MacPro1,1   MacPro3,1  MacPro5,1* MacPro6,1 
 1. FreeDOS            +/-         uc         uc         uc
 2. Windows 95          -           -          -          -
 3. Windows XP         ++          ++         ++          -
 4. Windows 7          ++          ++         ++          -
 5. Windows 8           -         (++)        ++         ++
 6. Windows 10          -         (++)       (++)        ++
 7. Slackware           +           +          +          +
 8. Debian              +           +          +          +
 9. Ubuntu              +           +          +          +
10. Arch                +           +          +          +
11. Gentoo              +           +          +          +
12. Fedora              +           +          +          +
13. Xinu                -           -          -          -
14. Minix              uc          uc         uc         uc
15. FreeBSD           +/-          uc         uc         uc
16. NetBSD            +/-         +/-        +/-         uc
17. OpenSolaris         +         +/-        +/-         uc
18. OS X v10.0          -           -          -          -
19. OS X v10.1          -           -          -          -
20. OS X v10.2          -           -          -          -
21. OS X v10.3          -           -          -          -
22. OS X v10.4         ++           -          -          -
23. OS X v10.5         ++          ++          -          -
24. OS X v10.6         ++          ++         ++          -
25. OS X v10.7         ++          ++         ++          -
26. OS X v10.8          -          ++         ++          -
27. OS X v10.9          -          ++         ++         ++
28. OS X v10.10         -          ++         ++         ++
29. OS X v10.11         -          ++         ++         ++

-: doesn't run  ++: officially supported (++): runs probably +: runs maybe one/two drivers missing
+/-: some drivers missing  uc: unclear  *Newer MacPro5,1s probably can't run 10.6 


I've compiled this list from various sources and it's probably not complete. I've embezzled the minor version numbers of the OS X versions (e.g MacPro1,1 needs at least 10.4.7). Some entries are well funded (like the Microsoft and Apple OSs), others are derived from my own experience or internet sources. A + or +/- doesn't necessarily mean that it is an easy job to get the system installed/booted. I welcome edits by more knowledgeable people though.



Addendum:

I've found two computer lookalikes which probably run all of those systems, the second one misses a video card though - and both are obviously no Mac Pros:





;-)