A lot of fears can be bundled into a "fear of the unknown" which is a common trait amongst humanity in general. That said, most of my fears have this sort of stipulation.
For example, Insects. A lot of people here have listed insects of varying types as a fear. Mainly this is due to not knowing how they behave, whether or not they're venomous, things like that. The more people understand insects, the less afraid they become of them. A good example of this is houseflies and mosquitos. Plenty of people are afraid of insects like spiders, but few have an irrational fear of flies and mosquitos, because you're pretty much forced to encounter them, and you learn that they're just a nuisance at most. My fear of insects works within these same bounds. If I recognize the insect, and know enough about it, they usually don't bother me (daddy long legs, roaches, centipedes, and so forth). Really exotic bugs I don't know, however, or bugs I know are venomous/painful, I give a wide, WIDE berth.
Another common fear circles around a lack of control over yourself. The most common example of this is falling, which I myself have a fairly strong fear of. I don't fall often, and I like to think I have a decent amount of dexterity to avoid falling in a painful manner. However, if a situation occurs where I am very likely to fall, or could somehow be moved involuntarily, I will shy away from it. When I feel comfortable with my knowledge of something, especially in regards to mobility (cars, bicycles, motorcycles) I usually have no problems with them.
I read somewhere that our minds are addicted to sensory input, so when we don't get any sensory input (for example, in a pitch black, quiet room) our mind will quite literally "make shit up" to feed on as sensory input, often resulting in hallucinations (either audible or visual). This also goes back to my initially mentioned "fear of the unknown" that we instinctively live by. These two factors combined usually result in a genuinely terrifying situation. However, if I have any knowledge of where I am, or what my surroundings are, than I'm usually fine. An example of the difference here would be as such: A power outage in my apartment wouldn't bother me at all, since I know where I am, and what's around me. Now, if I were to be rendered unconscious, and woke up inside a pitch black room with no sound at all for me to rationalize, I would likely start panicking very quickly.
I could go on longer, I think, but the summary here is this: The more you know, the less that scares you. This is where the common saying "the best way to get over a fear is to face it" comes from. Once you've experienced it, and conquered it, you'll understand it, and thus not be bothered by it again. I got over my fear of spiders ages ago with a combination of studying them, and actually interacting with them (held a tarantula, fuck yeah!).