Rhetorical rVoice is a professional speech synthesis system - they build voices for artificial call centres and the like. Their demo page is perfect for generating, say, an earnest Spanish gentleman rapping 'Bring The Noise' [100k mp3], or a stoned valley girl doing 'Night of the Living Bassheads' [160k mp3]. If you're hunting for Radiohead/ Kraftwerk/ Hawking robot voices, then you'll have to look elsewhere.