zeet

Realcalc has an excellent RPN mode (enabled through Calculator Type in Settings). I’ve used it for the last six years or so (technically, in the premium version of the app).

Edit: Not open source - sorry, didn’t see the community!

I use BetterOpenWith. It lets you set defaults or gives you a selection of apps to choose from for a wide variety of file types / protocols:

https://play.google.com/store/apps/details?id=com.aboutmycode.betteropenwith

You think a Gentoo user would appear in a comic with a graphical interface?