The Excel file format converter is a free application on Google App Engine powered by Aspose.Cells for Java. It allows you to upload CSV, Excel 97-2007, ODS, Spreadsheet XML and TabDelimited files from your local machines and then converting it to any of these excel formats; CSV, ODS, Excel 97-2007, Excel2007, Excel2007 XLSM, Excel2007 XLTM, Excel2007 XLTX, Spreadsheet XML and TabDelimited files. The conversion is very swift and you can save the converted file back to your machine using the application.

You can view the application here