No matter which scuba diving agency you train through, once you get to the leadership level there are some basic theory requirements you must complete (the Recreational Scuba Training Council requires it). With areas ranging from physiology to physics, it can feel like a lot of material.
Fortunately, Rod Abbotson of Dive Aqaba has put together a Dive Theory Study Guide. It's PADI-centric in that it is a (very quick) summary of their Recreational Encyclopedia, but I imagine even those from other agencies would find it useful in their studying, given the significant overlap in material. It is especially useful by the time you get around to instructor courses and have been tested on the same material 3-4 times. I used it to quickly find those areas where I needed to read the original text.
It may also be of interest to others, like open water divers with an interest in science as it relates to diving. Or question writers for Jeopardy.
Here is his version in Microsoft Word format. I've converted it to PDF as well: