How are you about respecting an unsolicited confidence?

For example, if a friend (not necessarily close) or acquaintance discloses a personal or sensitive opinion/fact/event and then tells you to keep it secret, do you honor that?
I’m not talking about a secret where someone could be physically harmed, or where a child is involved, but of the “Don’t tell my husband that I bought an expensive widget” or “Don’t tell my sister that I know that she had an affair” variety.
How bound do you feel to keep a secret you did not agree to hear?