KSpread

Example screen KSpread

KSpread is a spreadsheet application that is part of KOffice, an integrated office suite for KDE desktop. Some KSpread features include: multiple worksheets within a single document, various possibilities formatting, support for more than 100 functions, templates, diagrams, spell checker, hyperlinks and sorting data.
KSpread document is an XML format, compressed with zip. However, KSpread has the ability to import spreadsheets from other formats, including Microsoft Excel, Spreadsheet APPLIX, Quattro Pro, CSV and OpenOffice.org Calc.