The most common form factor of PC Cases are:
ATX
Micro ATX
Mini ITX
Other form factors which are not as popular are:
AT
Baby AT
Mini AT
Flex ATX
NLX
Mini NLX
In the near future a new standard will be used called:
BTX
This will be for Intel based PC's
'Via' wil also be bringing a new miniature form factor called:
Nano-ITX