Just yesterday we posted on the bc command line calculator. Well on the other end of the UI spectrum is Soulver from Acqualia Software. Its authors claim that Soulver is a "plain English" calculator in that you can actually type in certain (in fact very limited) phrases and get answers, as you see above. In general, though, you'll use Soulver as a normal sort of calculator, but with a pleasing, easy to read interface. The expressions to be evaluated go on in the left column and the answers appear in the right, all in a nice clean list. As you can see, you can also open a couple of palettes with conversions and statistics on the entire column. Perhaps ironically, the actual number crunching power under the pretty GUI is the same bc mentioned above. Soulver is not meant to be a particularly sophisticated scientific calculator (though you can define variables, for instance), but rather a kind of "back of the envelope" replacement. It's an unusual concept, but once you get your mind around the idea of it not looking like a stand-alone calculator, I actually think the paradigm works well (and I put my own money where my mouth is on this one).
Soulver is $17.95 and a demo is available.