I regularly do about 4 or 5 extra hours a week but I get paid for it so it doesn't bother me one iota.
Although when I've had jobs in the past where no overtime is paid I'd still do a little bit here and there if it was desperately needed and helped out fellow workers. As long as it wasnt taking the piss i didn't mind.
Years ago (nearly 20) I worked for a certain well known advertising agency and was clocking up nearly 60 hours a week without overtime pay (contracted 37.5 hrs) because in those days, in the industry it was considered 'the thing to do' so i naively went along with it. Safe to say it's moved on a long way since then, at least in my experience. Horrendous piss take when I look back.