See SatoshiLabs' blog posts about this feature:
- TREZOR Firmware 1.3.4 enables SSH login
- TREZOR Firmware 1.3.6 — GPG Signing, SSH Login Updates and Advanced Transaction Features for Segwit
- TREZOR Firmware 1.4.0 — GPG decryption support
See the following instructions for the TREZOR, Keepkey and Ledger Nano S devices.
For SSH, see the following instructions (for Windows support, see trezor-ssh-agent project by Martin Lízner).
For GPG, see the following instructions.
See here for PIN entering instructions.