GNOME Office's Spreadsheet.
Gnumeric. Numeric. Get it?
Gnumeric looks a LOT like Microsoft Excel. It also handles Excel format file (.xls) very well, apart from embedded Visual Basic stuff.
It also handles many other formats, and can save in a wide variety of formats, including its own XML-based format (which results in much smaller files than Excel), HTML, and LaTeX.
A nicely polished Linux application.