I had not given a whole lot of thought to bicycle hand signals until was driving a motor vehicle today (gasp!) following a bicyclist who suddenly indicated with his arm and hand that he was turning right. When he did this I immediately knew that he was as old as I am because he indicated his right turn with his left arm and hand. My second realization was that I had not added a page to this web site about how the bicyclist should signal to other drivers, bicyclists, and pedestrians. So here it is.
There are some other signals that come in handy when out on your bike.
If you are being followed by another bicyclist, it is always polite to point our a hole in the pavement, glass on the roadway, a dead skunk, a large limb that would be dangerous to ride over, or a myriad of other nasty things in the road by pointing at the hazard as you come up to and roll past the object.
When riding along in one direction, and you see an approaching rider on the other side of the roadway heading in the other direction, it's always nice to give a nice low down wave, not taking your hand off the handlebars very far.
Finally, there is one hand signal that you should probably refrain from using, although you are probably tempted to use it on a daily basis, this is the interplanetary recognized one finger salute. You just never know how the person receiving that salute will react, but almost never is it a positive response. If you feel the urge to communicate to others in this way while on your bicycle, think happy thoughts instead - the other person probably doesn't even know they did a stupid thing.