This is a simple server which will listen for events when window focus changes and allow you to focus the last focused window. This replaces the majority of my use of Alt+Tab on other window managers; ie quickly switching between two windows.
Download the compiled binary from the releases page, or build it yourself. There are no dependencies outside the Go standard library.
Add a line to your i3 config to start the server automatically.
exec_always --no-startup-id path/to/i3-recents
Add a line to your i3 config so your chosen key will trigger the client
bindsym Mod1+Tab exec path/to/i3-recents --back