I use Spreadsheet Writer a lot and it's nice to have, but if you don't need multiple sheets or the ability to do some formatting, etc, you an always just output HTML and send a
header("Content-type: application/vnd.ms-excel")
and an extension of ".xls" and then Excel or OpenOffice's spreadsheet program (and I assume, I guess, Mac's spreadsheet program) will open it and format the HTML as a spreadsheet.
It's not a native solution but in the end, it looks pretty good to the end-user. You have to use tables in your html but you can add colors/borders, padding, font changes, rowspan, colspan, etc, for a bit of control. No functions or formulas or multiple sheets, though.