If someone invites you to do a gig, then unless its for charity or some other good cause, and they're charging people to come and listen, then of course you should be paid.

If you do a floorspot at a club, whether by invitation or just turning up and asking, then you are shouldn't expect to get paid. You are supporting a club which probably couldn't afford to pay professional performers for a full night; you are possibly promoting yourself so that you willnget invited to do paid gigs; you are gaining experience and boosting your ego. That should be enough.

Sessions are just that, a chance to play together. If the audience want to put into a hat, or buy you a drink, that's fine. I disagree with the comment that the facilitator should get paid - usually this consists of nothing more than passing the word round at other sessions, and it doesn't go down well with musicians who are playing for the crack and the beer to learn that one of their number is being paid.