I tried to reply to http://news.ycombinator.com/item?id=2939370 but I think that thread is archived so I'll submit a new thread, despite being only a few weeks old.
In a nutshell, OpalCalc is a Soulver-like calculator for Windows (.NET 3.5 or higher), which is as easy to use as Notepad++ or a word processor.
OpalCalc allows natural language in the sum and multi-line support so you can go back and adjust your previous calculations on the fly. You can insert variables and create custom functions as well as tons of other stuff (the link includes a video and images which shows a lot of its functionality).
I hope this software is of interest to many of you, as it is a 'first of its kind' for Windows.