So, you have someone grubbing around in your game, looking for fancy things with which to stock his components pouch/herbalists satchel/what-have-you? Good! I have some charts. They assume a 2e D&D ruleset, but with a little tweaking they should be useable with whatever rules you like.
After one hour of herbalism use, roll on the following charts for results. If the herbalist searches for multiple hours, roll again. Any time the same result is rolled, double the previous amount (e.g., if the herbalist searches for 3 hours and rolls feverfew three times, it is no longer a small patch; hour 1 = 1 batch of tea, hour 2 = 2 batches, hour 3 = 4 batches).
1. Small patch of feverfew. This is enough to brew up one batch of feverfew tea, which is helpful in disposing of migraines and headaches. It may cut down on any penalties associated with sickness (particularly those to Intelligence or Wisdom), and may help resist mind-control effects for the next 1d4 hours (granting a +5% resistance to all charm-related spells or powers).
2. Thistles. This is enough to brew up one batch of milk-thistle tea. It will allow anyone drinking it to take a new save against any sickness they may have, or grant a +2 bonus to any save in a sickness-related save within the next 8 hours.
3. Handful of meadowsweet. This is enough to, over the course of 3 hours, make into one meadowsweet ointment. It must be mixed with a lard base and boiled. The ointment is useful for dulling pain and, when used in conjunction with first aid, will return an extra hp of health to the person being treated.
4. Bees! The herbalist must save vs. breath weapon or take 1 point of subdual damage from bees. If the herbalist saves, they may approach this bees' nest safely if they have any way to produce smoke and collect honey. While it has no medicinal purpose, it sure tastes good, and can be used to brew mead.
5. Flowering Melissa. This herb can be used to flavor food, and generally has a calmative effect that combats melancholia. Any spell causing emotion saves at a +1 bonus for the next 8 hours.
6. Horehound. Enough of these flowers to brew one pot of horehound tea. It is useful for dispelling head colds and chesty coughs, and grants a +3 saving throw bonus to resist or recover from sicknesses of that type for the next 1d4 days.
7. Mugwort. Used to treat cramps, bloating, and sore feet. Can be made into a mugwort ointment, which permits travelers to push themselves for an extra mile per day without effort. This is enough to make one application, which is good for one traveller for one day.
8. Rosemary. The flowers can be used to brew an all-purpose remedial tea (granting a +1 bonus on all sickness-related saves for the next 1d4 days) or be reduced to a pomade by boiling with lard, which makes a face wash (which can be sold for 1sp). Rosemary wood can be burned to lye-bearing ash to make a mouthwash and teeth-cleaner.
9. Rue. Enough rue to treat one venomous bite. When using rue on poisoned wounds, the herbalist may grant a +2 bonus to the target's save vs. poison. It is, additionally, a common ingredient in holy water.
10. Sage. Can be used to prepare a purgative, which grants a +4 bonus to saves vs. poison when the poison has been ingested. Can also be used to brew one batch of sage tea, which grants the imbiber +1 effective Intelligence and Wisdom for the purposes of NWPs for 1d4 hours.
11. Thyme. A key ingredient in anti-infection incenses and fumigations, thyme can be burned to help ward off disease. This is enough to burn for 30 minutes. Any area under the influences of thyme-based incense fumes for 1 hour will grant a +1 bonus to all disease and sickness based saves in the area.
12. Yarrow-root. Used to help treat battle-wounds and infectious bites. Wounds treated with yarrow-root pomade heal at +1 hp per day. Treating an infectious bite with yarrow pomade grants a +1 saving throw bonus to the target's save to resist the disease.