Anything by Kon Satoshi right now can be considered Superflat.
Like Paranoia Agent, Paprika... that sort of thing.
Also, Hideaki Anno and the more chaotic days of Gainax, so Kare Kano and Eva, that sort of thing.
You might also try Yuasa's Mind Game, although I'm hesitant to count that.
I don't really know what to make of Supeflat to be honest. While I love Satoshi Kon and Anno, I don't have much to say about the likes of Murakami.