Coursera

I have finished two of their courses and dropped in on a couple others. Some of them are better/more rigorous than others. I still prefer edX, however.

As for your signature track question: I can't imagine it carrying much weight with employers. However, it might increase the likelihood that you will actually finish the course instead of stopping to watch the videos two weeks in. If so, it might be worth the money just for that.

I did a couple in big data analytics. The material was pretty good, though very much a "jump in and see if you can swim" experience. I would assume the verified certificate is worth about the paper it's printed on.

I'm currently taking Calc 1&2, Statistics, Intro to Logic and Modeling Engineering Systems on this. So far they've all be really high quality.