The Emacs Calculator (2009)
Being an old HP48 user, finding Calc was what me fall the rest of the way in love with Emacs. If I'm at the shell prompt I'll still startup GP/Pari for quick calculations, but it's hard to beat Calc for longer sessions.<p>The only downside is that, like any advanced Emacs functionality, if you don't use it frequently you forget all the commands.