To also contribute a plant myself:
My current favorite is Jiaogulan. Even though it isn't really a spice, it is used to make tea (very mild) and they say it has the power to keep you young (not sure how much and if there is anything really to it).
I found it by browsing the stock of a very good German herbal/medical plant merchant.
I got very intrigued by the description and just bought one plant and got to get a second plant from it by propagating via cuttings :D
It adds a light citruy fresh taste to the tea. I personally always mix it with some other tea, because by itself is very mild. I most often drink it with a basic green tea, some ginger and honey.