KSpread - Open Source Spreadsheets

KSpread is a fully-featured calculation and spreadsheet tool. Use it to quickly create and calculate various business-related spreadsheets, such as income and expenditure, employee working hours.

WikiCalc - Open Source Spreadsheets

wikiCalc is a web application, created by Dan Bricklin, that allows for the creation and editing of spreadsheets through a wiki-style user-editable interface. It is currently released as version 1.0 for use on Windows, Mac, Linux/Unix, and other platforms that can run the Perl language.

Gnumeric - Open Source Spreadsheets

Gnumeric is a spreadsheet program that is part of the GNOME Software Desktop Project. Gnumeric is distributed as software under the GNU GPL license; it is intended to replace proprietary and other spreadsheet programs such as Microsoft Excel. Gnumeric has the ability to import and export data in several file formats, including CSV, Microsoft Excel , Microsoft Works spreadsheets (*.wks), HTML, LaTeX, Lotus 1-2-3, OpenDocument and Quattro Pro; its native format is the Gnumeric file format (.gnm or .gnumeric), an XML file compressed with gzip.
Gnumeric's accuracy has helped it to establish a niche for statistical analysis and other scientific tasks.

Gantt Project - Open Source Management

Open Source Project scheduling & Management

Gantt Project is a cross-platform desktop tool for project scheduling and management. It runs on Windows, Linux and MacOSX, it is free and its code is opensource. Use it to define project deadlines and milestones, assign tasks, track resource allocation, and more. Charts are saved as .PNGs and can be generated in HTML and PDF or exported to Microsoft Project. GanttProject doesn't have all the bells and whistles of other mind mapping software apps, but it does take some of their most important elements and puts them front and center for the user.