They're not all the same. It doesn't really matter which one you use as long as you read the directions on how to use it and follow the directions.
I prefer soapcalc.net, but some prefer the calculator on Brambleberry or The Sage.
I also like summerbeemeadow because it is easy to resize based on the dimensions of your mold, but I ALWAYS enter anything I do on soapcalc, I like getting my measurements in grams and ounces.
No, not all online lye calculators are created equal, most are good but make sure you use one of the larger established ones with a good reputation, not JoeHacker'sonlinelyecalc or whatever...