For me it was Capsela and Gakken Denshi Blocks:

http://en.wikipedia.org/wiki/Gakken_EX-System

Denshi blocks were one of the most significant educational toys that I had as a kid growing up in the 70's and 80's, and many an afternoon was spent with my Space 1999 Eagle model, a Capsela "moon base", and me "inside with the computer" (Denshi blocks) commanding 'the system'. Ah, to return to those halcyon days when modular toy systems gave my systems-management skills a sharper edge. ;)