The peace lily is an attractive calming (IMO) plant and cleans the air! I found this website which lists the top 10 houseplants for cleaner air indoors.
I also like armardi's good suggestion of a herb plant.
Try lemon verbena... like armardi says if you rub the leaves you release the aroma, and lemon is supposed to sharpen your mind!
or lavender is recommended for relaxing.... not tried it though, so I could be talking rubbish.