While most sail plans include the obvious sails such as a mainsail and a jib or genoa, there are other sails to consider! Some of these sails will end up being useless and others more useful than expected.
By far, the most useless sail to invest in is the spinnaker while the most useful sail is the trysail. The spinnaker is a large unruly downwind sail that no one ever flies. It takes a competent crew to set and douse it, and if anything goes awry, it happens quickly and severely. Out of fear or laziness, there is always an excuse not to fly the spinnaker.
Now for the most useful sail: the trysail! This little sail is sold as a storm sail, but it has so much more potential than that. We use our trysail whenever we are sailing downwind, light or heavy airs. The trysail doesn't use the boom, so accidental jibes go from a fear to a nuisance. Being such a small sail, it won't produce sufficient weather helm to bring the boat up into the wind when paired with a well sized headsail, such as a 100% Jib, Genoa, or Drifter. Best of all, the trysail keeps its forces on the boat within the deck.
What I mean here is the trysail is set on the mast and sheeted to the aft toe rail. As you ease the sheets while going downwind, the sail will fill up its belly next to the mast within the spreaders. The clew will simply move forward and not outboard. This means that the Center of Effort (CE) is kept midship and merely moves forward, not out.
A third reefed main is about the same size as a trysail, but set on the boom. As you ease the reefed main, the boom will move the clew very far outboard of the rigging and deck. The result is a long lever arm aft of the mast that is pushing the yacht to weather. This will create a lot of weather helm and may begin to struggle against the headsails lee helm.
When sailing downwind, the last thing you want is weather helm. You want to go downwind, and so should your sails. Instead of spending money on a downwind spinnaker you will never use, consider buying a trysail and adding it to your downwind sail plan.
One last point, you might be wondering why even fly the trysail since it is such a small scrap of cloth. The answer is simple, it adds sail area. Albeit not very much, but more than you had before you hoisted it up!