Both OpenOffice.org and LibreOffice (a derivative of OpenOffice) provide free open-source software packages that are popular alternatives to Microsoft Office. They are both very good tools and highly compatible with Excel.
Vertex42 focuses on Excel, so people may not know that we offer 100+ free templates for OpenOffice and LibreOffice as free .ods file downloads, including calendar templates, schedules, financial calculators, budgets, invoices, checklists, and many others.
OpenOffice.org and LibreOffice use the OpenDocument Format for Office Applications (a.k.a. ODF), an XML-based file format. What does that mean? Well, for spreadsheets, it means that the files are going to end in the .ods extension. Documents like letters and resumes use the .odt extension.
If you are looking specifically for OpenOffice templates or LibreOffice templates, browse Vertex42 and look for the button or link on Vertex42's template pages labeled "OpenOffice," such as the button shown in the image on the right. These will be .ods files.
If we DON'T have a separate .ods download, then 9 times out of 10 the Excel file is going to open just fine in OpenOffice and LibreOffice anyway.
If you find a template that you'd like to use and there isn't a .ods download and the Excel file does NOT work in OpenOffice, please let me know (you can comment below or shoot me an email).