Hanging in your cycling wardrobe (you do have a wardrobe just for cycle clothing don't you?) should be a gilet. They're an essential bit of clothing that comes into use in many a situation, and increase your clothing flexibility a great deal.
Ask any four-season cyclist if they own a gilet, and those that do will fill you in on why it's such a useful and versatile piece of kit. Firstly it has no sleeves, so unlike jackets you don't get over heated or feel restricted when you're working out really hard. Generally the front panels are windproof so your core stays protected from the wind and the back panel allows out heat and moisture, keeping you warm but not hot.
You can layer up according to the temperature and use the gilet as a windstopper for your torso, with your arms out so temperature can be better regulated. On especially chilly days they can be worn underneath a jacket for added thermal and windproofing protection. They really are a year round piece of kit and being so lightweight they can be teamed with other clothing really nicely.
Chosing a gilet
As usual prices vary considerably depending on the make and fabric. Some fabrics are simple and offer basic windproof properties but if it showers may not provide enough protection. Well known fabrics such as Windstopper will protect even when it gets wet and offer some water resistance too. Note that if they have mesh backs they're not the ideal choice for rainy weather riding, but in an emergency they'll help keep you warm on the way home.
When you're trying one on in the shop, wear a couple of layers underneath to check you can still feel comfortable when the temperature cools down. Go for a close fit that allows you to breath freely and try bending over to check the rear is long enough to cover the small of your back in the riding position. Especially important if you ride on the road.
You should expect to pay more for pockets, fleecy lined collars and storage pouches etc, also check to see how small it packs down depending on what use you expect to give it.
More advice on which gilet to spend your cash on, take a look at the member reviews for hundreds of reviews by RCUK readers.