What's a handy terminal command you use often?

tldr is great. Basically a crowd-sourced alternative to man with much more concise entries. Example:

$ tldr dhcpcd

  DHCP client.
  More information: <https://roy.marples.name/projects/dhcpcd>.

  Release all address leases:

      sudo dhcpcd --release

  Request the DHCP server for new leases:

      sudo dhcpcd --rebind