[billybek]

Angela is bang on.

A couple of other things worth mentioning.
If you are passing a hiker/jogger on a wide trail in the same direction, don't sneak up on them and ring your bell or call "on your left". When you can see them, talk to them from a distance so you don't surprise and scare them.
If passing in opposite directions knock your speed down so bad things don't happen. Especially if you are rocking a sweet downhill. It just makes you look like you are not an inconsiderate dick when you are passing them at a reasonable speed.
Initiate a little conversation. Being on a bike (as we have seen in other threads) seems to dehumanize us in some way so talking to others on the trail system reminds them that we are people out to enjoy nature too.
Horses.... They win hands down. Give them lots of room and announce your intentions and move off the trail if required.

Edit: As someone else had mentioned, the uphill rider on a narrow path has the right of way.