hi, i made a thread about this awhile back . i ordered some indian almond leaves from ebay to put in my tank. you need to make sure the leaves are not toxic to your tank, some i know that are not toxic are the indian almond leaves that i mention and also walnut and oak(dont quote me) leaves from what i know. the leaves have to be completely dried out.
pros:
they have some beneficial bacterial and anti fungal properties
add natural decor to your tank, reducing stress to fish
provide food for certain fish
great for brackish tanks
cons:
releases alot of tannis making your water brown (con if you dont like this)
will eventually decompose and disappear so you have to 'renew' it eventually
can get sucked into your filter