I keep forgetting this line…

$ cat ~/.ssh/id_dsa.pub | ssh user@host 'cat >> .ssh/authorized_keys'

Update – you might instead want to try:

$ cat ~/.ssh/id_rsa.pub | ssh user@host 'cat >> .ssh/authorized_keys'

If it still doesn’t work, check the file permissions: 0700 on the .ssh directory and 0644 on the authorized_keys file are working for me, 0664 on authorized_keys isn’t.

Thanks to Mathias Kettner – SSH login without password.

Update (2021-01-17): Easier, and just works (which the former method doesn’t anymore on my latest server):

$ ssh-copy-id user@host

Update: See Gert van Dijk – Upgrade your SSH keys!

Update (2017-06-19): For a while, it seemed one should use .ssh/authorized_keys2 instead. Someday I’ll take the time to actually learn about this…