MSFT excel viewer http://office.microsoft.com/downloads/2000/xlviewer.aspx

header("Content-type: application/ms-excel; name=file1.xls"); 
header("Content-Disposition: inline; filename=file1.xls"); 
echo "a1\tb1\t\na2\tb2\t\n";exit;

PHP Excel Class without needing COM support: http://psxlsgen.sourceforge.net

NOTE: There is a big difference between viewing XLS files as a MSFT works SpreadSheet and a MSFT Excel SpreadSheet. Basically, one can't do the other.