Translation:The weather is good today, isn't it?
よく is an adverb, the corresponding adjective is よい or いい (it is handy to remember that いい is originally よい when you conjugate it).
Excuse me, would you please elaborate "remember that いい is originally よい when you conjugate it". I don't follow. Thanks ^_^
「今日は天気がよいですね。」is the original sentence, which is used for writings and formal situations. In conversation you'll hear 「いいですね」 a lot more often. 「よいです。」is the opposite of 「わるいです。」
いい is an irregular adjective
いい - it is good いかった - it was good よくない - it is not good よくなかった - it wasn't good