Spoutcraft is a modified Minecraft client that allows for custom content. It basically allows us to do everything that we're doing. It's still quite buggy, (think early Minecraft days) so don't be surprised when strange things happen from time to time. (Mobs lose their skins a lot right now, for example.)
Still, the client is pretty cool, and gives us some fresh new content that we so desperately need.
No installation is necessary. Just run the client exe (available below) and you're good to go; just like Minecraft. It sets itself up in your application data folder, but it creates its own folder as not to disturb your normal Minecraft client data. It can be used to connect to normal Minecraft servers too, though.