Rosegarden is an open-source MIDI and audio sequencer and notation editor that offers a huge range of features, an active developer and user base and a rich long history.
Richard Bown of Xyglo was one of the original developers of Rosegarden and now maintains a windows port. This has more limited audio and MIDI functionality but still offers a impressive range of features for notaton, MIDI editing and some playback and recording. You can download Version 14.02 (June 2014) from the sourceforge download page.
Rosegarden is still actively maintained having first been open-source licenced in 1995. You can head over to the main Rosegarden site for more information about the core development and the user community.