gpg-edit-recipients is a companion-tool to GnuPG to add recipients (technically-speaking Public-Key Encrypted Session Key packets) to an encrypted file. It is able to do so without decrypting the whole file, and without losing existing signatures.
Such a tool is very useful when you change your encryption key, if you don't want to keep your old key forever but still want to be able to decrypt old files and messages. It is also useful to forward an encryped message to someone else.
gpg-edit-recipients is currently in alpha status. All comments and suggestions are welcome.