Which cryptocurrency doesn't use blockchain?

Which cryptocurrency is unique in the sense that it doesn't rely on the tried-and-true blockchain technology? This question piques my curiosity as I delve into the vast and diverse world of cryptocurrencies. Blockchain, after all, has been the backbone of numerous digital currencies, providing a secure and decentralized ledger system. So, which one stands out as an outlier, forgoing this fundamental technology? I'm intrigued to know more about this outlier, its underlying principles, and how it manages to operate without blockchain, given the significance of this technology in the cryptosphere. Could it be a revolutionary new approach or a niche experiment? Either way, I'm eager to unwrap its mysteries and understand its place in the crypto ecosystem.