Cobblestone / Stone Generator

This one's older than most Minecraft players. Flowing water meets flowing lava, you get cobblestone. Keep the lava as a still source and let water flow into it and you get obsidian instead, which is why the classic generator design is careful to keep the lava flowing, not static.

Manual version: dig a short trench, lava at one end, water at the other, let them meet in the middle. Mine the cobblestone that forms, and the flows touch again on their own the moment the block is gone. No redstone. Just repeated mining.

The automatic stone variant is fussier. If water and lava sources touch on the same game tick at a spot a piston can reach, and you push a block into that exact junction while the water's still moving, it forms stone instead of cobblestone. A repeater-timed piston handles the push, shoves the new block out to a break point, and the sources stay put to do it again next cycle.

Either version is really about the same thing: an AFK-able source of a block you'd otherwise mine by hand. Good for building, and if you're running furnaces nearby, cobblestone into stone is about the cheapest smelting XP loop there is in the early game.

It also happens to be the one design on this page that works exactly the same in the Nether, since water and lava are both common there. Handy if you're setting up a Nether base and don't want to ferry cobblestone in.