Emperor Palpatine hand-picked and trained a small group of operatives, the Emperor's Hands giving them just enough training in the Force to help them carry out their missions. After the Emperor's downfall, most of them went out of their way to make trouble for the Alliance, but one of them (Mara Jade) ended up marrying Luke Skywalker.
Wookieepedia has an article on Dark Side Adepts that lists a number of people and organizations that used similar ideas (along with a few actual dark Jedi).
As for using such people for good, I don't recall any, and it would make less sense; dark Jedi would want groups not trained enough to pose a threat to them, but that's not usually a concern for light Jedi.