Awesome
tmux-cpu-info
Shows a tiny bar in your tmux statusline with the current CPU usage. Works on OSX, might work on other platforms.
Example
High Usage
Medium Usage
Low Usage
Could also show memory usage with some slight customization.
Installation
Add the following to ~/.tmux.conf
set -g status-left '#(tmux-cpu-info)'
Or for a more complete statusline like the one I have