OverlayPlugin allows you to display different overlays over your game and supply them with data from ACT.
Information about ACT and FFXIV
For more information on ACT, take a look at their website.
For a list of popular FFXIV overlays, take a look at this list.
If you want to stream your overlay over OBS, take a look at the streamer infos linked above.
If you want to write your own overlay, take a look at the developer documentation.