Have you seen Brad Lidge's "slider" or at least that's what they call it. His slider comes out on straight trajectory, as if it was a fastball, but breaks hard and late down and away to righties. A "true" slider has more sideways movement than downward movement. Some pitchers' sliders just have different movement depending upon the axis that the ball is spinning on.
Francisco Rodriguez, closer for the Angels, also has a similar looking "slider". It looks more like a curveball actually. Lots of downward break. He throws it around 80 mph.