I'm a big fan of using cordless drills to speed up tasks whenever possible. This trick from The Manly Housekeeper takes the idea to a new level. Just mod the top of a typical hand-held scrubber with a drill bit attachment, snap the assembly together, and get to work.
The video shows you how the final product works, and the full step-by-step (complete with photos) is at the link below. It's pretty clever, since you don't have to keep the drill around to use it -- you can still use the brush by hand if you want, and attach it to the drill when you know you need some extra grunt. The author puts the drill bit into the scrub brush, but uses a nut driver attachment on the drill to attach the two magnetically. He note that for the best results, you really have to keep the brush and drill perpendicular to the surface you're cleaning. If you start to angle it at all, the scrubber could pop off, and you'll have to reattach it, since it's just the magnet holding the two together.
You could use a proper bit attachment if you want it to be a bit sturdier, or pick up a Drill Brush, as one of his commenters suggested. In any case, hit the link below to see how he did it.
How to Clean Your Bathtub the Manly Way [The Manly Housekeeper]