Catnip (Nepeta cataria) is the one I use most often that is safe (unless you drink a lot of it and plan to drive). It calms the nervous system and has slight sedative properties. Don't use it if you are pregnant though as it is an emmenogogue (it affects the female reproductive system).
As mentioned above chamomile (either Roman or German) helps relax the body. Be careful using it if you are allergic to anything in the daisy family. It should also have an apple smell to it. If you don't smell apples, don't use it as it is old and has lost it's medicinal/healing properties.
Peppermint (Mentha piperita) is also helpful for relaxing the body to aid in sleep.
I make my own teas and frequently drink a combination of the above herbs just to relax, and for the other health benefits.
Valerian (Valeriana officinalis) is another that is effective but about about 5% of user have a reaction to it, normally in the form of hallucinations. It should only be used in small amounts in blends and only for a short amount of time with frequent breaks.