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For quick calculations I just use spotlight or alfred, whichever I happen to launch. For more complex calculations I use Soulver.

Soulver is very similar to a project I started working on a few months ago. I guess I didn't do enough market research!

I think there is still place for similar projects. Although do look at Calca as well if you have not done so yet.

In regard to both, I suppose that there is an opening to something more complex.