Retro Boy is a new free single-oscillator synth from SBAudio that's designed for creating chiptune-style sounds. It comes with seven selectable waveshapes and various other controls.
With a design that's said to be 'lightweight', Retro Boy shouldn't tax your CPU too much (it supports up to four voices), while the interface is very simple. A bigger, more feature-packed XL version is in the works, though there's no release date at the moment.
You can download Retro Boy as a Windows VST plugin from the SBAudio website.