XLS Editor for Google Chrome is an offline and free software that you can use to edit Excel documents, either in.xls format (which you can open with Microsoft Excel) or.xlsx (which you can open with OpenOffice). It allows you to add, remove, change, format and otherwise manipulate the columns and rows of the file and, importantly, does not require you to use the Internet in order to work. You can also work with multiple documents and it allows you to add images, charts and other stuff to your files and to keep all your data in sync across multiple documents.

If you prefer to edit your documents using the software you already use, you can do so using this app. It can also open a.xlsx document and keep it synced with the original one.