One of the most useful items I have ever purchased is a set of deflectors for my Rotalux softbox. This simple set of accessories converts my softbox into so many different light sources.
With a combination of the 2 diffusers and the 3 deflectors I can get anything from the exquisitely soft light with both diffusers to the crisp light of a beauty dish - I use mine all the time!
In the following examples only a single Rotalux 100 Octa was used with the various accessories, so you can see exactly what these do.
Sorry this is only possible with Elinchrom lights due to their unique central umbrella tube.