Quadrature!
The End-Of-Rise gives you a trigger when the terminal value of an EG's rise is hit. So, let's set the four rise/fall parameters to something that approximates a triangle wave from an LFO. Next, connect the EOR on #1 to the Trig input on EG #2, and so on until the #4 EOR loops back to #1's Trig input. With this, each EG outputs a waveform that's delayed by the previous EG's arrival at the EOR point, which results in a 90-degree "phase" shift between each EG's output. And once you've triggered this system once, it'll keep doing that until hell freezes over!
Why? OK...consider this one: take your quadrature outs and mix the 0 and 90 degree together, then do the same for the 180 and 270 degree EGs. Send the resultant mixed outputs to a pair of, oh...let's say Intellijel Polarises...control inputs. Now you have a pretty bizarre bit of stereo phasing going on, almost acting like an autopan but no autopan could sound THIS nuts! Note that you have to use the 90-degree angles here; combining 0 and 180, etc. will just result in phase cancellation.
As for EOF, it's super-useful for chaining EGs to create a complex composite modulation signal. One ends, starts the next, etc. Then just send the result through a DC-coupled mixer and you get super-varied LFO behavior. Or you can use the EOF in some generative functions to fire off the "next" set of parameters, and as long as there's a feedback loop (not acoustical feedback, mind you) that can keep right on going, and other modules can then modify the individual EG parameters depending on which EOF has sent a trigger. You could actually wring two signal paths of this sort out of a single Quadrax/Qx!
Right now, I'd say that the Quadrax/Qx combo is one of the best module sets in this regard, but the newer Tiptop/Buchla 281t also can do this...as it's got built-in quadrature outputs for the A+B and C+D outs.