Apple's icon container, the thing inside every .app bundle that makes it look like something. Like ICO it holds a set of sizes, but it goes considerably larger - Retina displays ask for 1024px icons, which Windows never did.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop an ICNS file above and choose afterwards.