Casascius physical bitcoins
Casascius physical bitcoins, also called Casascius coins, are physical metal coins created by Bitcoin user Casascius (Mike Caldwell, Sandy, Utah, USA) that contain an embedded piece of paper with digital Bitcoin value, covered by a tamper-resistant hologram. Casascius coins are available in 1 BTC and 25 BTC increments. They can be purchased at Casascius's website, https://www.casascius.com (only Bitcoin accepted), or at http://www.MemoryDealers.com (PayPal and credit cards accepted).
The coins are designed such that they could be circulated in face-to-face transactions. The first person to redeem a private key gets the value on the coin, and afterwards, the coin no longer has any Bitcoin value. However, it is difficult or impossible to read the private key on the coin without damaging or destroying the hologram, which exposes a honeycomb-like tamper evidence pattern when peeled.
The piece of paper inside each coin has a private key which forms the backing for the Bitcoin value represented by the coin. Redeeming the private key back into digital Bitcoins is currently available at MtGox (via the Add Funds - Redeem Private Key screen), as well as with a patched Bitcoin client.