Kiwix is an offline reader for Web content. It's especially intended to make Wikipedia available offline.
With Kiwix, you can enjoy Wikipedia on a boat, in the middle of nowhere... or in Jail.
Kiwix manages to do that by reading ZIM files, a highly compressed open format with additional meta-data.
Canorus is a free cross-platform music score editor. It supports an unlimited number and length of staffs, polyphony, a MIDI playback of notes, chord markings, lyrics, import/export filters to formats like MIDI, MusicXML, ABC Music, MusiXTeX and LilyPond
ZeGrapher is a free and open source software for plotting mathematical objects (functions, sequences, parametric equations) and experimental data on the plane. This software is intended to be as easy to use as possible, suitable for high school students.