I really like Calca and still use it a bunch, but it has a few annoyances (some of which cause incorrect answers) and seems to get updated about once every 4 years.

Putting the answers inline rather than over in a separate column (ala Soulver and Numi) makes more sense to me.

Calca is absolutely incredible and an invaluable part of my workflow. I only wish that praeclarum would open source it (with a prominent option to donate) so that it can live on forever, since it seems a little undermaintained at the moment. As far as I know, there's nothing exactly like it out there (with respect to typing calculations in plain text, then immediately seeing results inline).

I'm using Calca since seems like there is no real alternative on Windows.