A little project I made recently (for a contest held by NIST):
The SciCalc-9000
It's a calculator, but it also has buttons for universal physical constants. And units. That way, if you want to figure out the energy of a photon of light that is 400nm, you can do it. (And get the answer in J or eV, your choice!) Basically, it combines the steps of *looking up constants* with the step of *plugging them into your calculator*.
I was surprised I didn't find (good) things like this in the app store already, so I spent a week or so and made this prototype. The link above is to a web build of it, though it's designed as an app. Figured it could fit in alongside the slide rule in my suite of apps.
Not sure exactly how far I'll take this one, though I imagine at the very least I'll polish it up a bit more and release it. If it looks like the type of thing you might use, by all means throw some feature requests my way - my goal is that it be useful for students / scientists. (Without getting *too* complex and huge. You can tell my bare-bones goal from the fact there's no way to add or subtract, since the first few uses that popped into my head didn't need them.)