How to get grass on the sides of grass blocks in Minecraft?

That's a setting in the Optifine mod, it's also included in some resource packs like Squidmaster said.

http://www.minecraftforum.net/...much-more/

Its a simple mod, also included in MCPatcher, called "Better Grass".
Its also included in a lot of the better Resource Packs.

It's probably a mod, or maybe a bug, as grass does not naturally grow on the side of dirt blocks.

You flip it