The Sith order doesn't literally require its members to be evil; It merely states how they access and use the Force. This raises the question if there are any Sith that can be described as intentionally doing "good" (Darth Sidious occasionally did good things as side effects of his plotting and bigger evil plan)?
We know that Master Windu (as well as Luke Skywalker) stepped really close to the Dark Side, while remaining still technically Jedi. I'm thinking about clearly crossing the line to the Dark Side and officially being a Sith, but still intentionally producing some positive outcome.
In this context, "good" should be subject to any reasonable interpretation of the word. For instance, enslaving half the galaxy (and killing the other half) "to finally have peace" (Sidious quote) is certainly not "good", although Sidious may have been convinced it was.
Edit: Regarding the good/evil (moral/immoral) discussion in the comments. This is Star Wars lingo. The language of Star Wars (in contrast to reality) dictates a more or less black and white picture on morality and therefore "good" can be considered as being an action, according to the Star Wars definition of "good".