We took 4 different available calculators and compared results obtained for Zo on a PCB microstrip. Results were that the SPG package and APPCAD results were very close, TXLINE results were larger than the SPG and APPCAD results, while the wCalc package was lower than the SPG and APPCAD numbers. (wCalc does provide some very valuable parameters as a result of the simulation: R, L, C and G parameters for the microstrip). Here are the comparisons.
Please visit the SPG website for more information and reports.