Digital Guitar Tuner is a free Windows software, which let's you accurately tune your guitar using direct input, microphone or MIDI tones. Now a new version 2.3 is available for downloading. Now it features 27 tuning types!

Digital Guitar Tuner Features
High accuracy (uses 16 bit / 44100 Hz input!)
You can tune guitars with no signal output (through microphone)
27 different tuning types (standard, lowered, dropped, open tunings, Hawaiian etc.)
Modulation of root frequency
Tuning with midi tones