While handheld lasers are useful and fun, they are all too often misused. This can cause very serious damage and it is strictly illegal in every country.
High powered lasers can cause PERMANENT and COMPLETE damage to the eye if one stares directly into the beam, even from long distances. Reflections from mirrors, windows and shiny objects also pose a threat. Never use lasers near such objects, and NEVER, EVER point your laser towards or even close to other people or animals. NB: Although laser safety glasses are designed to protect against laser beams, please do not consider them 100% safe. There are always a chance that they don't block enough light or that the laser melts the glass and shines through.
People aiming laser pointers at aircrafts or vehicles are a growing problem all over the world. Few people realize exactly how bright the laser is perceived by the driver or pilot. A laser hundreds of meters away can cause major distraction or even eye damage, and therefore very dangerous situations. It's illegal in most countries. In the US, pointing a laser pointer at an aircraft has a penalty of up to five years in jail.
Another hazard of very high powered laser pointers is the fact that it can heat up objects to extreme temperatures and cause fire and burn damage. NEVER aim your laser at flammable or sensitive material.
More information can be had at laserpointersafety.com.