By far the best GUI calculator I used is Qalculate: http://qalculate.sourceforge.net/ http://qalculate.sourceforge.net/ It's ridicolously fast and lightweight

By far the best GUI calculator I used is Qalculate: http://qalculate.sourceforge.net/ http://qalculate.sourceforge.net/ It's ridicolously fast and lightweight for the amount of features it contains, and it's also free and open source.