What are the most common, useful, herbs in your world?

I also want to know what they can be used for.

Ooh, thanks for posting, this is giving me ideas!
A common weed called Hare's Foot, something between a thistle and a dandelion, the raw sap when rubbed on an insect bite can relieve the itching, and the leaves when cooked briefly can be eaten like spinach.
A plant called Changeling Cane, similar to dumb cane (Dieffenbachia) or taro, a tall leafy stem plant that grows abundantly along creek banks and in wet soils. The stem and leaves are not to be eaten, they cause pain and swelling like dumb cane, but the small tuber when prepared correctly is a popular recreational psychoactive hallucinogenic ;)

That sounds interesting

Possibly the most useful in mine is an herb called Luowell and it's for medicinal purposes. The sap inside of it has to be prepared in boiling water, but afterwards it can heal either breaks or fractures. It's extremely painful when used and can cause some side effects, such as nausea, fevers, nightmares, excessive sweating, and/or severe tremors.
Luowell plants are rather tall, but the stems are kind of thick and circular. They grows tiny flowers from their tips that have no real purpose. Most also have heavy, dark leaves that sprout from their base, which are a type of vegetable used in healthy dishes.

The most common useful herbs would probably be the ones that pertain to mythology and legends. Cultures will glorify or dramatize the uses of a common plant for supernatural purposes oftentimes, and if you dig deep enough you can come up with some pretty interesting real-world elements to tie into your fictions. One I use is the use of Wolfsbane to kill wolf-creatures. It was said that the Greeks used to marinate rotting meat in wolfsbane to leave out and kill wolves off.