I take quite a few regularly in my food. Not only to make food taste better but I do believe they can have beneficial effects and lots of them have been well tested. I tend to choose them to increase my ability to battle the cold and help fight off colds and infections.
Garlic I rate highest. Great taste, an immune system booster, makes proteins easier to digest, thins the blood and makes a fantastic topical painkiller. Nothing works better (except for the dentist or doctor) for severe toothache than chewing raw garlic. Roman soldiers used to eat raw garlic before battle-its gives a bit of a boost and may have helped with fighting infection from wounds.
Cayenne pepper-proven to help the body grow new blood capillaries, boosts the metabolism.
Black pepper-helps digest proteins, increases metabolism.
Ginger-anti bacterial, anti viral. Also attracts insects-not so good.
Parsely-another metabolism booster
Turmeric-anti-bacterial. Good for sore throats.
Rosemary-reduces cholesterol levels in fried. An anti-oxidant too-helps keep bread a little fresher if its made with it.
Chocolate/cocoa-another metabolism booster and great for soothing a sore throat.
Cinnamon-plays a role in beneficially regulating blood sugar levels.
Thyme-wards off Trolls. It hasn't worked on this thread though.
There are many more, many more effects. I've just listed the well known ones that I reach for first and I believe work.
There isn't much downside to them-even if they have no beneficial effects, they still taste nice