The Cosmic Cannon is first used by Vegeta against Goku during the Saiyan Saga. It was a fireball-like ki blast that can burn his opponents. He used it to burn off half of Goku's shirt. He used it a second time but it proved ineffective against Goku's Kaio-ken.
Cell used it once in the Cell Games Saga. During his fight with Goku, Perfect Cell used it to destroy the Cell Games arena after becoming annoyed by its size limits.
It can be assumed Vegeta has evolved this technique into his much more powerful technique, the Big Bang Attack, but this is not proven.