People always mention Frink and GNU Units, but the workhorse of desktop unit-aware calculation is, in my opinion, Qalculate. In addition to being a capable scie

People always mention Frink and GNU Units, but the workhorse of desktop unit-aware calculation is, in my opinion, Qalculate. In addition to being a capable scientific calculator in its own right, its killer feature is the ability to throw 'x' anywhere into an equality, and it will solve for 'x'. AS well as making to/from unit conversion even easier this also often saves you having to rewrite physics equations to isolate the quantity of interest (and thus potentially screwing up factors when eg converting mm^2 to m^2). The composition of units + symbolic calculator is greater than the sum of its parts.