Voyager uses a RTG. The USSR used some full-on nuclear reactors in space, but as far as I know no one else publicly did.
Edit: Yep, not sure how I forgot about that.
Edit: Yep, not sure how I forgot about that.
Edit: it’s apparently still there and will be for a long time (!) albeit non-functional:
> Decay date: April 3, 5966 (planned)