Biopace, since 1988.
But on shimano, the biopace was on the bad place : try to turn the rings on the spider : small diameter when the arm is vertical, and big diameter, when the arm is horizontal (when you got max strength)
Having missed out on the original Biopace craze I rode my first ovalised chainringed bike last year. I find it very similar to regular round rings. The only noticable difference is when using the smallest chainring, things get a bit choppy. I like it.