μFade module. This is another Swiss Army Knife of a module, since it can act as a mixer, crossfader (it's primary stated purpose), VCA and weird-o waveshaper.
I haven't had this nearly as long as the LAMP-1, but that doesn't matter - I fell in love with it the second it came in the house. I have it sitting right next to my two modded Pittsburgh oscillators, and it is my oscillator mixer as its everyday module.
But where it really gets fun is when you CV-control whatever it is that is running through the A and B channels. Having two oscillators tuned to the same (or similar) frequencies gives you an alternative way of adjusting harmonic content without filtering. But it gets even more fun when you start pushing crazy waveshapes and wavesequences through this module, then using a not-quite-normal LFO waveshape (I'll often use two mixed sawtooth LFOs running at different speeds) to drive the mix setting. You can get massively complex sounds without having too many modules dedicated to the task.
The big modification of my μFade was the replacement of the V1 knobs with a set of V2 knobs (Intellijel μModules are all on a version 2 except for this one...); this makes it fit in with the other Ijel modules I'm obsessed with, and the whole thing is wonderful!