Qalculate! is a multi-purpose cross-platform desktop calculator. It is simple to use but provides power and versatility normally reserved for complicated math packages, as well as useful tools for everyday needs (such as currency conversion and percent calculation).
This may be unremarkable to others, but I was really excited to see that Qalculate! can solve for x. I really miss my TI-89 graphing calculator and having something that can solve complex equations for me is comforting.