Open Source has a philosophy of developing and sharing software for free use. Some of this, and other free-to-use, software is useful for use in school. Here's some free-to-use software we've used:
AndreaMosaic - Make mosaic pictures using your own photographs.
GameMaker - Programming software.
GIMP - The power of Photoshop, for free.
Leonardo Sketch - A vector based drawing application.
Scratch - Well known programming software.
TUX Paint - Paint for mac, Windows & Linux.
UPDATE 29/11/14:
PosteRazor - Make posters from images.
Tux of Math - Maths game. (via @MrLiconti)
GCompris - Educational games for children up to 10 years old. (via @MrLiconti)
Triptico - Interactive resources.
Inkscape - Drawing software.
UPDATE 29/11/14:
PosteRazor - Make posters from images.
Tux of Math - Maths game. (via @MrLiconti)
GCompris - Educational games for children up to 10 years old. (via @MrLiconti)
Triptico - Interactive resources.
Inkscape - Drawing software.
Sourceforge can provide a place to search for other software. Let use know if there's any other you know of that we've not included.
Note: this post was updated further on 21/01/14. Thanks for the comments.