What file suffix is usually used for the filename following a -o?

Pick your favorite:
- nothing, which is not the only option but also not uncommon for output 
from a compiler
- .vvp (or whatever the target might be) because it helps you to 
remember what it is and what to do with it
- .out because it is closest to the default a.out

I've seen all of those options.

However, I do not use any suffix if possible. It's a couple of extra 
keystrokes either way and I am lazy. :)