If you're working on an Excel spreadsheet in the office (for example), you can save it to a memory stick and then open it with OpenOffice Calc (for example) at home. (OpenOffice Calc recognises Excel formats). You can then continue working on it at home, with OpenOffice Calc, just if you were using Excel.
By default, OpenOffice Calc saves spreadsheets using OpenDocument format. (i.e. .odf, rather than .xls or .xlsx). You can get around that on a one-off basis by using 'Save As', instead of 'Save' and selecting an Excel format. Alternatively, if you want OpenOffice Calc to
always use an Excel format, you can change the default format by following the instructions under 'Default file formats' here:
https://wiki.openoffice.org/wiki/Documentation/OOoAuthors_User_Manual/Getting_Started/File_formats