Eephus: The eephus is one of the strangest pitches in the game because the goal is to throw it as slow as possible. Not like a changeup, which is intended to be slow in relation to a pitchers fastball, the eephus has a high arcing motion. For this reason it has been alternately named the moonball (popularized when Bill "Spaceman" Lee threw it), the Bugs Bunny curve, the gravity curve, the rainbow pitch and the blooper. The effectiveness of this pitch was typically due to batters not expecting it. Rip Sewell (who pitched in the '40s) reportedly was the first to popularize the pitch. There have been some modern day players to use the pitch, but not with any regularity: Vincent Padilla, Steve Hamilton, Mark Buehrle, Dave LaRoche, Casey Fossum to name a few. (Eephus is though to have been named after the Hebrew word 'efes', meaning 'nothing'. Outfielder Maurice Van Robays is credited with naming the pitch.)
Knuckleball: The knuckleball is a trademark for a number of pitchers. Typically, those who throw the knuckleball do so almost exclusively (R.A. Dickey, Charlie Hough, Tim Wakefield). Despite the name of the pitch, most throw it using the tips of the fingers instead of the knuckles. The goal when throwing a knuckler is to put as little spin on the ball as possible. The lack of spin results in a very erratic and unpredictable path to the plate. Most who throw the pitch claim it doesn't put much stress on the body since speed is not a goal so many players use the pitch to extend their careers or will try to learn the pitch to remain effective after injuries. Most pitchers do not try to learn the knuckler simply because making mistakes with the pitch results in a very straight ball that is often hit very hard. Charlie Hough once lost a 5-hitter (7-4 final score) and all five hits were home runs. In spite of the difficulty associated with learning and mastering the pitch, there are always a few pitchers in the game throwing it.
Screwball: The screwball is considered by many to be a 'reverse curve' since it uses the opposite twisting motion of a curve. This results in the ball breaking from left to right from the perspective of the pitcher when thrown by a right-hander. Baseball historians (such as Wayne Stewart) tab Hub Pruett (pitched in the '20s) as the first pitcher to find real success with the pitch. In his time Pruett was known for his ability to be trouble for big hitters such as Babe Ruth and Ty Cobb. Screwballer Carl Hubbell often said it is best to throw if the batter is looking for a fastball since it is the change in speed that results in a swing and miss. Not many players throw this pitch currently or throughout history but some big names have: Fernando Valenzuela, Dallas Braden, Warren Spahn, Juan Marichal, Mike Marshall, Pedro Martinez, Mike Cuellar, John Franco.