The Comet is a sportscar based on the Porsche 911. The Comet is all-wheel-drive like many of the newer models, yet looks most like a 911 from the late '70s or early '80s. First Featured in GTA VC, also in GTA SA.
The Comet has also been spotted in a screen capture for GTA IV. It now appears in a hardtop coupe form, instead of it's usual targa-top configuration. However, it retains it's Porsche-like silhouette, although now appears have a design influenced in part by a BMW Z Coupe.