OpenKeychain is now maintained by Cotech.
OpenKeychain is the most widely used OpenPGP implementation on Android. With OpenKeychain you can easily generate new key pairs and manage the public keys of your contacts. With its great integration in K-9 Mail there is a user-friendly way to send PGP/MIME emails. Besides this, you can execute cryptographic operations, such as signature generation and encryption, manually in OpenKeychain.
My favorite feature is the support for security tokens.