DrZhark's Farmcraft Minecraft Farming Mod
You know you've done this before. Painstakingly built a fence and shoved a few sheep or pigs or cows inside it and trapped them there and pretended that you're farming them. Unfortunately, the default version of Minecraft makes animals despawn, so overnight your animal farm is little more than a series of small, desperately empty pens and you are left feeling strangely bereft, as if your world is fake and means nothing at all.
Farmcraft changes that (and very possibly makes your life better) by allowing you to domesticate animals by dropping food near them. Animals domesticated (or tamed) this way will not despawn, which means you can finally keep animals in the pastures and pens you build for them. Huzzah!
That's not where the joys of this mod end though, Farmcraft takes the farming experience further in a myriad of ways. Sit a while and I will relate them to you.
Once your animals are tamed they will follow you if you carry around a shepherd's crook (a craftable item). Cows will drop manure, which can be used to craft items like flower seeds, rose seeds and pumpkin seeds. You can also craft six bundles of wheat together to create what the modder calls a haystack and what I call a hay bale, because I'm super agricultural.
You can also breed animals by keeping the animals you want to breed at least six blocks away from other animals (apparently they're nervous breeders) and feeding them appropriate food. Cows, pigs and sheep need hay to breed, chickens need seeds.
Farmcraft adds much needed functionality to the game without making it feel a wee bit odd (like some other mods, that add Pegasuses and whatnot tend to do.) That's not to say that mythical creatures aren't awesome and shouldn't be in Minecraft, they definitely should be in Minecraft. Definitely.
Farmcraft uses TFC's Mod Manager, which is not a bad little mod managing tool that will probably gain popularity in time because unlike Mod Loader, it actually works. If you don't like using mod loading mod managing tools you can just download the basic version and toss the class files etc. into the minecraft.jar the old fashioned way.
Be aware that this mod doesn't seem to work at all unless one also installs the Mo'Creatures mod, which adds horses, werewolves, lions and other creatures to the game. Without Mo'Creatures, the mod installs but everything appears to be invisible. You can craft Shepherd's crooks until the cows come home, but you won't be able to see 'em that that's not awesome at all.