A calculator with units. Prerequisite: pip install peglet.
What it can do:
>>> from unitcalc import *
>>> load()
>>> calc('3000 furlongs / fortnight')
0.49892857142857144 m s^-1
>>> calc('(2 * 6 ft * earthradius)^.5 in miles')
2.9990780906608245 # The horizon from a 6-foot-high POV is 3 miles off.
You can calculate from Python code without converting your formulas to strings:
>>> hour, day = calc('hour'), calc('day')
>>> Quantity(7) * day / hour
168.0 # Hours in a week
The obviously nicer 7 * day / hour wouldn't be hard to support.
Its knowledge comes from definitions.units from GNU Units. Since it was a hacky 4-page hack, much of GNU Units didn't make it in, but thousands of definitions useful to technical calculations did.
The code conceives of a value-with-units as a polynomial whose variables are the basic uninterpreted units, such as 'm', meter. (This means Fahrenheit and Centigrade aren't units it can know about, except as temperature differences -- they have different zero-points.) A slightly cute bit of implementation: it uses my parsing library Peglet to code just a scanner, with the actual parsing done by precedence climbing.