If you save an Excel file as an HTML file (File->Save as Web Page), you can check "Add Interactivity" so that the person can work with entered formulas and then click on the "Export to Excel" button which will copy everything to a new Excel file. The other option would be to save the Excel file, just as you normally would and put a copy of it on the web site, so when someone click on the link, it will just open in Excel (of course they need Excel installed on their computer and unless you know that they do I certainly wouldn't assume this).
If you are trying to use forms online I would create them in HTML with JavaScript and any necessary server-side scripts to email the data back to yourself so that the end users do not need to have Excel setup.