Jonas Beskow at the Centre for Speech Technology KTH Stockholm wrote free Formant Synthesis Demo computer programme that runs on Windows and Linux (and on any other OS for which the application can be compiled from the open source code the author kindly uploaded).
The programme synthesises F1, F2, F3 and F4 formants from several sources (rectangle, triangle, sine, sampled and noise). It “demonstrates formant-based synthesis of vowels in real time, in the spirit of Gunnar Fant’s Orator Verbis Electris (OVE-1) synthesiser of 1953” (from the About window).
„Formants are defined by Fant as ‘the spectral peaks of the sound spectrum |P(f)|’ of the voice. Formant is also used to mean an acoustic resonance, and, in speech science and phonetics, a resonance of the human vocal tract. It is often measured as an amplitude peak in the frequency spectrum of the sound, using a spectrogram (in the figure) or a spectrum analyzer, though in vowels spoken with a high fundamental frequency, as in a female or child voice, the frequency of the resonance may lie between the widely-spread harmonics and hence no peak is visible. In acoustics, it refers to a peak in the sound envelope and/or to a resonance in sound sources, notably musical instruments, as well as that of sound chambers” — Wikipedia.
The download link is on the Formant Synthesis Demo site.