There is an obituary (in German) here. Reader Alexander Stingl, who kindly called this to my attention, also wrote a nice summary of his contributions:
Apel was, as your readers may know, an influential voice in bringing American Pragmatism, especially Peirce's ideas, into the German philosophical discourse. Furthermore, he created a unique conversation between the ideas of Peirce, Wittgenstein, and Heidegger. He was a proponent of Transzendentalpragmatik and Discourse Ethics which was analog to and for many years in conversation with the version of Habermas, yet also distinctly different, e.g. in Apel's consideration of the possibility of a "first philosophy". Apel remained an active writer, his most recent papers published just earlier this year in German by the respected publisher Suhrkamp, where a majority of his works has appeared, including the the two-volume collection of essays Transformation der Philosophie from 1973, which many consider his finest and most influential work. (Also, the English Wikipedia site, which many may consult now, is, erhhh, quite embarrassing).