Minor suggestion that may help you get more responses... they way you phrased the question with the word "on" is a little ambiguous. I can't tell if you are asking which people/companies are investing 'in' side projects of other developers or if you are asking if there are individual developers investing their own time/money working 'on' their own side projects (and perhaps you're curious about what they're working on to potentially collaborate). If the former I recommend using 'in', if the latter I would use 'on'.
I'm not a grammar cop, I just literally could not determine which question you were asking.