Knights are usually good cards and can totally destroy the opponents deck when you can play many of them, but there are lots of counters
-Mini Decks like Golden Deck are awesome (just beware of Sir Michael, it's best if you can trash it with another knight or buy it on your own). In general, VP tokens are nice against Knights
-Hamlet-Poor House (little Deck sizes and cheap cards, plus provinces you cant trash)
-Potion-based Strategies (SP-decks, maybe uni, philosopher stone, golem) vineyard not so much since you can destroy a huge amount of the action cards, if they dont cost 1-2$, possesion and familiar obviously don't counter knights too
-Reactions like moat or secret chamber deserve honorable mention when you can hold them in hand (almost) every turn, and naturally when lighthouse in the kingdom a knights-only-strategy should be ignored. market square is also worth a consideration to get gold for the trashed cards.
-I dont know if Colony can counter Knights. Platinum cant be trashed, but if you just have colonies and platinums you have to draw 3 of the per turn and your opponent will still win by building a solid engine.
-Strategies where you can ensure that your key cards are immune against knights (by holding them in and every turn or placing some VP on mats, NV/Bridge)
-It's mostly good play if you buy sir vander against a knight player. You don't want to hurt your opponent much but you can trash him one of his valuable knights and even get a gold (which you will probably lose afterwards, but hey, it's a gold for free
-Benefit-when-trashed cards are nice but dont totally counter knights most of the time. Fortress is one of the best in that categories. You can build up an engine of fortress+draw+X and overbuying the fortresses a bit, to make your opponentmore likely to hit them with the knights.
-Gainers like IW are not totally counters, but they prevent you from being destroyed. They give you middle-range cards ($3-$4) which don't hurt that hard if you have to trash them (already mentioned, here in general as "Silver floods")