If you run Minecraft on a Mac, you've probably noticed that things in the distance look horrible! Clouds look stair-stepped, snow tears into the block beneath it, torches pop in and out of walls, and so on. This is because by default on a Mac, Minecraft uses an 8-bit depth buffer.
When rendering a 3D scene, the depth buffer stores the distance from the eye point to each pixel in the scene. If a new pixel (called a fragment) is rendered that is further away than an existing fragment at the same location on the screen, it is thrown out. This is why you can't see every block on the map at once, even though they're all rendered. An 8-bit depth buffer only allows for 256 levels of distance--not a lot. As a result, fragments in the distance get rounded to the same depth value, even though one may actually be in front of the other. When multiple fragments share the same depth value, the most recently rendered one is the one you see, even if it's not actually the "frontmost" in the scene. This is why you see parts of objects show through in front of the things they're behind.
This mod increases the size of the depth buffer to 32-bits per pixel, resulting in dramatically improved image quality. It only changes one line of code. I tweeted @notch several times asking him to make the simple change, but he hasn't yet. If you want to help encourage Notch to make the change, you can retweet my tweet.
Compare the difference in quality between the two images below. If you see the glitches shown in the first image, this mod is for you.
The alternative to this patch is to use a patched version of lwjgl which sets a 32bit depth buffer. This means that point releases will still work, only if Mojang upgrade the lwjgl library will the fix stop working.