Giving the windiness that accompanies biking outdoors, you may want an extra layer over your torso for warmth, but without the bulk of a jacket. If that's the case, add a vest. Most sports apparel manufacturers make sports vests that work well for biking.
Depending on the conditions and your preferences, you can find vests in wind-resistant fabrics such as Gore-tex, which work best as an outer layer, or puffer-style vests made of down or a down substitute. Down vests are bulkier than the breathable, moisture-wicking styles, but they are warmer and may be a good choice for the casual cyclist who is not pedaling as fast and getting as warm as someone who's out for a training ride. A fleece vest is another good layering alternative for cyclists, and a lightweight fleece vest often works well when layered over a long-sleeved moisture-wicking shirt and under a wind-resistant jacket.