Canned compressed air is sold in many electronics and computer stores as a cleaning item. It is used to blow dust off delicate or hard to reach components, and is also useful for cleaning optical equipment.
At $15 (US), canned compressed air is significantly more expensive than atmospheric air. Presumably this is due to the air having been filtered so as not to blow dust onto the components.
Canned compressed air comes with several warnings. It should not be shaken. It can be flammable. The initial spray may contain some liquid, and should be directed away from electronics and people.
Unlike the compressed air used in SCUBA diving, canned compressed air must never be inhaled. Inhaling canned compressed air can result in brain damage or death due to the toxic nature of the propellants used in the can. In fact, inhaling the propellant is illegal in some countries, and a problem among not-so-intelligent teenagers who believe permanent impairment is pleasurable1.
The most curious warning on the label of canned compressed air is that one should not insert the nozzle in the ear. Obviously directing a stream of highly-pressurized, extremely cold2 air into a delicate area of the body is a bad idea. Why anyone would want to do this is the curious part. There is no warning against blowing canned compressed air into your eyes or other body parts, however this too should be avoided at all costs.
Canned compressed air is both relatively dangerous and expensive when compared to the air we breathe. As it turns out, the air we breathe can be just as effective.
For cleaning electronics, try a small bicycle pump or small plastic bellows (available from camera shops).
At a pinch, swallow, take a deep breath, pucker and blow. Using your own lungs as a propellant is the cheapest and easiest method, and is perfect for doing some rough cleaning of things like keyboards. However, like canned compressed air, the first spray may contain some liquid, and should be directed away from electronics and people3.
If you want air blown into your ear, try asking a friend or lover to inhale, pucker and blow. This is considerably more enjoyable than canned compressed air.