Gnumeric is a spreadsheet application included as a part of the GNOME Office suite. It supports a slew of different formats, including Microsoft Excel, CSV, OpenDocument, and LaTeX, just to name a few. It copies most of the features available in the more ubiquous, proprietary spreadsheet programs, including charts, random number generation, statistical analysis, styling, and batch processing.
As many of you know, I work at a job that requires me to use Microsoft Excel by force. I strongly dislike Excel. Therefore, I'm proud to announce that Gnumeric supports 100% of the worksheet functions in .xls documents, and has 154 functions not included in Microsoft's program. The Gnumeric team is working on adding support for Pivot tables.