The Tortuga Effects Abduction has two different sets of old-stock Germanium diodes at its core that add a depth and color not available from most overdrives.
The SkyChord (King Capitol Punishment) SleepDrone 5 has 4 square wave oscillators.
The MXR M77 includes 100HZ cut and boost control that allows for a more focused EQ when cut, or a beefier tone when boosted.