I wasn't aware of them either, until I did a quick search. I mean, I'm not really among their primary targets. And I do mean "quick". Though I'm a bit surprised by your answer as these offers are quite easy to find. I googled for "gratis sprachkurse für ausländer in zürich", doesn't seem to difficult, no?
Btw I also came across this one
. The Kulturlegi may be interesting to those with a low budget as it gets you 30-70% rebate on offers in and around Zürich. The linked page also features a "free activities" guide containing a very broad selection of offers.