Aquallegro is a great way to improve your musical ability. It contains quizzes on various aspects of music and detailed help files with tips and mnemonics. The quizzes are scored while in progress, so the user always knows how well they're doing.
Audacity is a free, easy-to-use and multilingual audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems.
MuseScore is a free cross platform WYSIWYG music notation program, licenced under GNU GPL.