Dominion - Saboteur use
I disagree strongly with aramis and goldPseudo's answers. Using the data from Isotropic, we can see that in all the games played on the site, people who bought 4 fewer Saboteurs than their opponent increased their win rate by a factor of 1.5. In fact, this graph suggests that the only worse attack card is Thief, and that even buying a single Saboteur hurts your odds of winning relative to someone who doesn't.

In my play group, I decided not to buy them at all, and I typically won when they were available. We've since decided that it's not a fun card, so now we don't play with it.

To quote the Dominion Strategy Blog, "The best counter to Saboteur is probably not getting one."