Offering the same workflow as its marvellous chipspeech plugin, Plogue's Alter/Ego is a new voice synthesizer that you can download for free.
Once you've typed in your lyrics, you can then play them from your MIDI keyboard, enabling you to create a synthesized vocal performance.
Said to offer a voice that's "not strictly vintage", the first soundbank for Alter/Ego is known as Daisy (this will also load into chipspeech). Further banks in different styles will be released in due course.
You can download Alter/Ego for PC and Mac in VST/AU/RTAS/AAX formats from the Plogue website.