Using other freqs to communicate between planes or ground?

I've wondered if it is legal to utilize an "unused" frequency to communicate between planes or
to someone on the ground for non-critical communication?
I don't know what for, but let's say you want to talk to your friend or CFI on the ground who
might give "additional instructions" on things.
Or, another pilot close by wants to exchange some restaurant info or something.
Or maybe a flying club wants to communicate or something.

Is that legal?
Is it ok?
(Let's assume your monitoring other freqs that you need to).