If you're a vim user, another useful tmux plugin is tmux-yank, which allows you to use the keyboard to navigate through tmux's scroll-back buffer (containing output from previous commands) using vim style keybinds and select and yank text to the system clipboard.
Tmux plugin for copying to system clipboard. Works on OSX, Linux and Cygwin. - tmux-plugins/tmux-yank
github.com
Useful for if you want to copy error messages to the clipboard, to paste into a search engine, or into a text editor, or another application, entirely with the keyboard, without having to resort to using the mouse.
Also, there's a book I recommend, called 'tmux productive mouse free development', which has some great tips for using tmux effectively.
tmux is a beast of a tool that I found to be indispensable for my personal developer workflow, but there are a few things that I have added to my tmux configuration to enhance the out-of-the-box experience.
Byobu is a GPLv3 open source text-based window manager and terminal multiplexer with elegant keybindings, status notifications, and profile management for GNU Screen and tmux.