I'm currently reading a book about various herbal teas and their medicinal properties, and it's very interesting! It's a short collection published by Microcosm Press called "Everyday Herbal Teamaking: A Pocket Guide for Health, Fun, and Self-Care."
Chamomile tea is the one that I'm most familiar with, it definitely has a calming effect on me and I find that a small amount of it added to other teas provides sweetness without any added sugars or honey. However, if I drink the more powerful loose-leaf variety that I get at my tea shop it actually has a numbing effect on my mouth if I drink it by itself! I'm not certain if this is everyone's experience, but it sure is interesting.
I've also dabbled with ginger teas, licorice teas, mint, etc. The more traditional herbal varieties, that's how I see them at least. The book has a lot of other options that I'm interested in trying out. They say that catnip tea is actually pretty tasty (the book has some qualitative reviews of each herbal tea as well), so I'm interested in trying it! Also from the book dandelion tea and ginseng tea sound appealing, and that's just from the first half that I've read so far.
The medicinal properties of these herbs admittedly have less appeal to me than just the flavor, but it's still interesting reading about what effects they supposedly have, as well as some of the scientific research that's been done on them. Overall, I'd recommend checking out the book if you're at all interested. I picked it up as part of a current Humble Bundle, but it's available in print or ebook form from Microcosm for a very reasonable price too.