The Shepherd is a free software synthesizer developed by Simple-Media. Available format(s): VST for Windows.
Kill Bill has a tune in its soundtrack by Ennio Morricone featuring a haunting and evocative whistling. This ‘mini-synth’ is designed solely to re-create that sound – just add reverb. This shepherd likes to whistle while he works.
http://www.simple-media.co.uk/vsti.htm