TW PitchShop is a free Versatile Mono / Poly Synth VST instrument with portamento.
Loads of controls to create (or adjust) sounds, divided over different screens. Easy to use interface, very understandable for beginners and experts. Interface ±840 pixels wide, height about 440 pixels.
Soundbank with 128 programmed sounds, some strange, some weird, not the standard soundkit.