ADSR-1A:

VC(R)ADSR - Design by EFM

This is one of my favorite EFM modules. Each parameter (A, D, S, R) is voltage controllable. It's great to route velocity through an inverter to the attack; this makes notes more punchy with higher velocities. This module also features a normal output and an offset output (which allows you to add a DC bias to the output signal). The best feature, however, is the 'Rate' control. This allows you to stretch or shrink the entire ADSR cycle while still maintaining the relationship between attack, decay and release times. Another nice touch is this envelope generator's ability to operate in AD (trigger), ADSR (gate) and LFO modes. There is a copy of the schematics available here

I have seen a modification to add an "End Output" which goes high at the end of the cycle. I'm thinking about adding this to one of my ADSR-1A's so I can use it as a gate delay, or I could cascade two EGs to overlap filter sweeps, panning etc.

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PEG-1A:

Dual Percussion EG - Design by Thomas Henry

This is a very simple envelope generator taken from the Electronic Drum Cookbook. It's a transistor buffer following a cap / resistor combination for a charging element. The design has a fixed (fast) attack and a variable decay. Since the design is so basic, I almost always favor the ADSR-1A. For this reason, the PEG-1A has been removed from the rack and retired. There is nothing "wrong" with the design, but I simply favor more flexibilty.