is up to v 0.9.3, there is a portable version (v 0.9.2) thanks to the WinPenPack guys (ie X-Launcher apps)
MuseScore is a cross platform WYSIWYG music notation program, licenced under GNU GPL.
Some highlights (from the Home Site):
WYSIWYG, notes are entered on a "virtual note sheet"
unlimited number of staves
up to four voices/staff
easy and fast note entry with mouse, keyboard or midi
integrated sequencer and FluidSynth software synthesizer
import and export of MusicXml and standard midifiles (SMF)
platform independent code, binaries available for Linux and Windows
GNU GPL licenced
More information is available from ...
The portable program is available from ...