What is the value of this capacitor?
Definitely 100nF (0.1&micro;F) and 100V

Considering the lack of other markings on the part, this is definitely a 0.1&micro;F (100nF), 100V capacitor; it is likely a monolithic ceramic type, but could be a film part as well.  It will work just fine for non-critical applications (such as decoupling), but I wouldn't use a cap like this in a signal path unless I knew more about it.

The other possibilities are 100pF and 0.1pF -- the former I can safely rule out as that'd be notated as "100 J 100" or something of that ilk instead, and 0.1pF is an insanely low value for a through-hole capacitor; a 0.1pF capacitor would get swamped by the parasitic capacitance of its own leads!