Weighing in at ~200,000 lines of code Gnumeric offers well tested import facilities from several proprietary and free spreadsheets including MS Excel (tm), Lotus 1-2-3 (tm), Applix (tm), Psion, Sylk, XBase, Oleo, and HTML. It can also export to MS Excel (tm) along with several open formats such as LaTeX \longtables, HTML, and roff.
Rounding out the i/o routines is a highly configurable text importer/exporter to ensure that data can be transfered smoothly. New formats can easily be added in a modular fashion via a plug-in.
Gnumeric has 361 spreadsheet functions, 17 analysis tools, and 2 solvers. The majority are MS Excel (tm) compatible, but there are several (eg number theory and financial derivatives) that are unique to Gnumeric. Most MS Excel (tm) functions are supported (298 out of 316). Although a few financial functions with insufficient documentation are still unimplemented. Contributions are welcome.