Casascius is primarily known for its physical bitcoins, which were collectible items that contained a real Bitcoin value and were created by Mike Caldwell. These physical coins were popular among cryptocurrency enthusiasts and collectors and played a unique role in the early history of Bitcoin. Each Casascius coin had a tamper-evident hologram containing a private key that could be redeemed to access the associated Bitcoin value. However, due to regulatory concerns, Caldwell stopped producing these physical bitcoins in late 2013. Casascius coins remain as historical artifacts of the early days of Bitcoin and are highly sought after by collectors.