Free Metronome:
Good software to learn to play in time. Uses actual
metronome ticking rather than beeps. Allows you
to set custom timing.
Download here
Powertab editor:
Creates, reads and plays guitar tablature and bass tablature
in the .ptb file format. Useful for learning difficult songs.
Download here
Tuxguitar:
A great tab editor and player that is able to use guitar pro
tabs. Has a full fretboard for guitar as well as piano keys
to show you how to play your favourite tabs.
Download here
ScaleTool:
Displays various scales and chords on either the guitar
fretboard or piano keys. A handy utility for learning
scales or chords.
Download here