CppSheets is a simple spreadsheet application that I work on as a hobby project. For more information about the project and the program itself, see the wiki.

Basic formatting (bold, italic, underline text; cell background color; text color; cell borders)
Cell merge
Custom XML-based file format
Basic xlsx (MS Excel) support
CSV loading support
Formula support (which is fairly good)
Basic graphing support

For more information, see the wiki.

Building is very easy; the only external dependency required is Qt5 (Core, Widgets, Gui). To build, simply run CMake and Make. Note: you should probably use G++ to build this program; I tried using Clang, and it failed while building the xlnt library (Update: Clang seems to be working now after a minor change in the xlnt library; this was only tested on MacOS though).

CppSheets is fully supported on Linux, Windows, and MacOS. It should work on other systems as well. I will list them here as I test them.

CppSheets is licensed under GPL-3.
The built-in TinyXML2 library is licensed under the zlib license.
The included xlnt library is licensed under MIT.
Qt5 is licensed under LGPL and GPL (the Qt Charts module).
The included Oxygen icons are licensed under LGPL.

See the wiki for more information.

As of version 2, CppSheets is now licensed under the GNU General Public License, version 3. Originally, it was licensed under BSD-3. The change has to do with the libraries; for graphing support, CppSheets uses the Qt Charts library, which is licensed under the GPL. To keep using this library, the license change was necessary.