Because OpenOffice.org is such an obvious and excellent replacement for Microsoft Office (including Word, Excel, and PowerPoint), it's easy to forget that there are other alternatives. One of the great things I keep coming back to when I talk and write about Linux is the fact that we do have alternatives, some costing no more than the time it takes to download and install them via Synaptic.
When it comes to spreadsheet programs, it's time to fire up Synaptic and do a search using the word spreadsheet or one of the following programs. The primary candidates are Gnumeric and KSpread.
Both Gnumeric and KSpread are certainly worth a look, but I've found Gnumeric to be particularly good when it comes to working with Excel spreadsheets.