Wiki-contributed follow-ups to the 9-awesome-ssh-tricks post:
Use autossh for managing persistent sessions/tunnels which will restart upon network or local issues.
OpenSSH >=5.4 will allow you to add new port-forwardings in multiplexing mode. If you start a new slave session with port forward requests they will automatically be relayed and added to the master. You can also request the mux master set up forwards without requesting a new session using "ssh -O forward -Rxx:yy:zz -Laa:bb:cc user@host"
Rather than grovelling though /tmp to find a working agent (which could connect you to a malicious one!), you might instead want to start the agent at a known location (e.g. ~/.ssh/auth_sock) using "ssh-agent -a /path/to/socket"
sshuttle - transparent proxy server that forwards over ssh, now you can have a full-featured vpn with security implemented by ssh. https://github.com/apenwarr/sshuttle
"CompressionLevel" is ignored in SSH v2 and higher. Supposedly the default is ideal.
"Cipher" is ignored in SSH v2 and higher. Use "Ciphers" and put your favorite on the front of the list. Type "man ssh" to see what ciphers are on your system.
Use arcfour (rc4) encryption for higher performance and very low load but be sure to enable re-keying by hour or by data volume. Rebuild OpenSSL and OpenSSH to include it.
Type "ssh -vvv email@example.com" for really detailed debugging information. More "v" means more verbose.
Try using "keychain" to discover, reap, and re-use those ssh-agents littering your system.
For seamless but secure remote execution: generate a new ssh key (ssh-keygen) without a pass-phrase; put the id and id.pub files on each "client" machine; add the id.pub to the authorized_keys file under the username used for the "server" end. Here's the trick: Insert restrictions before the public key, but all on the one long line. So instead of "ssh-dss AAAAB3blahblahblah..." in the authorized_keys, use "no-port-forwarding,no-X11-forwarding,command="/the/specific/command",from="client1.ip.addr,*.other.clients" ssh-dss AAAAB3blahblahblah..."