A fast, down-the-line serve from the backhand corner (normally done with a forehand pendulum service motion) is a valuable serve. It's great at catching righty forehand loopers off guard as they look to loop your serve from the backhand corner. It's also sometimes good against more neutral players, as they aren't used to fast serves to the forehand and so can be erratic against it. It's even effective against lefties, who are often guarding against serves wide to their forehand, and so are vulnerable to this sudden serve to their wide backhand. Against lefties, however, you'd look to ace them by going crosscourt to their forehands, though you are less likely to get an ace as players generally guard the crosscourt angle. (Lefty servers, this serve is effective for you as well - but I'll let you work out the specifics.) So develop this serve or you will be handicapping yourself against some players.
If you aren't sure how to do the serve, ask a coach or top player to demonstrate. But the key is to fake as if you are serving crosscourt, then at the last second change directions and serve fast topspin down the line. Contact point should be a bit behind the table so that the first bounce can be near your own end-line, which gives the serve the maximum amount of table to drop and hit the other side. (This is true for fast crosscourt serves as well.)
Pick and choose when to use this serve. Often you can tell by the receiver's stance and body language if he's looking to step around. Even if he's not stepping around, if a receiver looks a bit flat-footed, he's probably vulnerable to this serve.
Once you can do the serve effectively, it's time to make it even more effective. Here are two ways you can do that. First, you can use spin, either sidespin either way or extra topspin. If you do, make sure to put more power into the serve so that you don't sacrifice much speed for spin. Extra topspin pulls the ball down and allows you to serve even faster.
Second, contact the ball with the racket still aimed at the righty receiver's backhand, but hit it down the line with sort of a slapping sidespin (your racket going from right to left at contact). This is my favorite. Typically the first time I go down the line, I do it the "easy" way, changing the racket direction at the last second. Now they are watching for that last second racket change, so I don't give it to them. By keeping the racket aimed to their backhand, you catch them moving to cover that shot, and their forehand side is open.
Once you've done a fast down-the-line serve, the rest of the match the opponent is guarding against it, and guess what? All your other serves become a bit more effective.