RTS Overlay is a tool used to display build orders (BO) and match data for AoE2 (also available for AoE4 and SC2).
It is a purely manual tool (no AI reading your screen, no automatic step update). So, it is mainly a glorified Notepad, tailored for AoE2 BOs (ideal for players without a second screen).
buildorderguide.com is probably the largest AoE2 BOs collection (hundreds of BOs available for all playstyles).
With the new RTS Overlay update, you can now import any BO from buildorderguide.com with a single button click.
See a full demo in the video below (RTS Overlay installation, setup, BOs import and demo on a skirmish game).