BitterSweet is a very popular transient designer by Flux Software.
It has been around for many years and is widely used. A huge selling point is that BitterSweet is completely free to download and use.
In this video, I explain how the interface works and show you a little demonstration to show you what the plugin does exactly.
To install BitterSweet you need to download the Flux Center, which is an application that allows you to download and manage any software from Flux.