The MXR classic overdrive pedal has a hidden switch inside that changes to the Zakk Wylde overdrive. I've seen people mod their own switch for it, but I just drilled a hole
ZW44 Wylde Overdrive - MXR - Max Guitar – Max Guitar
MXR ZW44 VS. MXR ZW44: To go Wylde, or be Berzerk? - YouTube