In Smash Bros. Melee you can dodge once in any of the 8 directions while airborne. If you do it close to the ground you'll slide. Doing this when coming down from a jump is called wavelanding, while jumping and doing it before you even leave the ground is called wavedashing. It's kind of useful because it's a very fast way to move with some characters, and there's absolutely no frame delay before being able to attack. Here's a video explaining some of the "pro-techniques" for Melee.
I personally don't mind much. Some of the basic ones are actually kind of interesting to use, and it's not like most of us will ever end up playing against someone knows them all. It doesn't affect the gameplay experience of most players and gives the really hardcore fans more to do.