I have to agree with Moontanman. In addition to the the kinds of friction he mentions, there is also friction due to tidal forces of gravity. Every time the earth rotates in the gravitational fields of the Sun and Moon, it is stretched a little bit. This stretching creates friction in the earth, and robs the earth a tiny amount of its rotational energy.
The moon keeps the same face pointed towards the earth all the time. How likely is it that this is a coincidence? It isn't, because tidal forces have long since robbed the moon of whatever rotational momentum it might have once had.
See:
Does the moon rotate?
Likewise, although tidal forces from distant objects and galaxies are very weak, they nevertheless do exist, and rob the planets of some tiny amount of orbital momentum. As well, gravitational waves predicted by Einstein are created by bodies orbiting one another. This too slowly robs orbital momentum. So, even in the absence of the kind of friction Moontanman has mentioned, the momentum of EVERYTHING is always being robbed a tiny bit because of gravity.