dandelions help tomatoes.
Calendula roots help deter bad nematodes (there are GOOD nematodes, unless in your body) that love tomato roots.
Rose geraniums, even in pots, near rose plants, help keep june bugs/Japanese beetles away.
Elder trees supposedly "elder " other herbs, and teach them.
Too much nitrogen decreases essential oils in herbs...so don't plant near high nitrogen-feeding plants.
lavender and rosemary enjoy each others company.
Yellow dock's deep roots bring iron closer to the surface, and make iron more available in the soil around it.
Mints are good interspersed with squash--perhaps to deter the dreaded squash bugs with their yellow larvae.
These are off the top of my head without consulting my many herb books, most of which don't mention companion planting anyway...
Mints are good inter