Rearrange FX Utility is a useful tool for organizing your REAPER RX folders! Once you have set up some of your favorite FX into folders in the FX area of Reaper, there is no way for you to drag them up or down to arrange them in the order you want. RearrangeFX allows you to do that. This article from the Reaper Blog explains more about this new tool.
It’s great that if something’s missing from Reaper, people can try to fix it (those who are capable), but i don’t like it when it’s a functionality that should be in Reaper natively.