Lemon Balm - If you are having trouble with pollination, planting lemon balm in your garden will attract bees.
Dandelion - This herb has been known to increase the aroma of all herbs. So, planting this by other herbs such as lavender and bergamot could bring in more wonderful scents! You can also try planting certain aromatic herbs in an area that has a lot of dandelions growing already. It has been known to concentrate potash in it's body.
Stinging Nettle - This plant has a few benefits. If used as a tea, it helps strengthen plants and promotes growth. It also increases essential oils in plants. It concentrates potassium, sulfur, calcium and iron in itself.
Chamomile - Used as a tea, it helps fight damping off in young plants. It also concentrates potash, sulfur and calcium in it's body. Plus, it just smells good!
Valerian - This plant will heighten phosphorous activity in the surrounding area. If you are lacking phosphorous in your soil, try planting these along the edge as they are perennials. It also helps in the health and disease resistance in plants.
There are many more herbs that can benefit your plants. Half of the fun is experimenting what works and what doesn't. Plus, what a great homeschool project for your children to try out some of these themselves! Make sure you have a control group of plants that do not have the experimental herb in it's surroundings. That way you will know for sure if it makes a difference as being beneficial to your garden.
What has worked for you?