All waveforms can be synthesised by adding sine waves of different amplitudes and frequencies - this is called a Fourier series. This is the basis of FM synthesis as used in the Yamaha DX7 synthesizers which were very popular in the 1980's (more modern synths tend to rely more on adding and procesing sampled sounds). A single sine wave is the purest sound possible as it contains only the fundemental frequency with no overtones. Most synths (pure samplers aside) are capable of generating a reasonable single sine wave, but as has already been noted it sounds pretty boring.

Ian S