[dTal]

Qalculate is the best desktop calculator I've used. It has a command line version, qalc.

Qalculate takes unit-aware calculation to the next level by including an algebraic solver. You don't have to rearrange everything to put your unknown on one side - you can just chuck 'x' into your equality somewhere and it'll work it out. For example, to verify the what-if xkcd assertion that an A10's main gun will propel Randall's car up to 7mph in 3 seconds:

(60/s) × (x s) × (395 grams) × (1010 m/s) = (3500 lbs)(70 mph) ≈ x = 2.0754319 s

2 seconds! Maybe Randall's car is heavier than 3500 lbs. How much heavier?

(60/s) × (3 s) × (395 grams) × (1010 m/s) = (x lb)(70 mph) ≈ x = 5059.1879

More like a Dodge Durango. Unless he's accounting for friction.

You see how nice it is to be able to swap around the unknowns in an equality. Other great features are a large number of built-in constants, an astounding variety of built-in functions - it's practically a CAS - and a command history, which is where I pulled the above example from. It also has a plugin for Plasma's krunner, so you can do quick calculations right there in the dropdown. To summarize: Qalculate is the shit.