Recently I donated blood.  I happen to have 0- (negative), which less than 7% of the population have, but 100% of the population can use. 

You can donate red blood cells, platelets or plasma.  Donors with 0+ or 0- are great for red blood cells, A+,A-, B+, or B- for platelets and AB- and AB+ for plasma donations. 

I try to get to the local blood center ever 8 weeks or so.  It doesn't take very long, it doesn't hurt and it is safe.  Anyone over 17 years old, weighing more than 110 pounds can donate.  There are a few exceptions, which the blood center will help you with.  Things like recent tattoos, piercings, and medicines you currently take.  The technicians (which are really, really nice!) will take a health history, take your vital signs, take your blood and then they give you snacks.  Works for me! 

Think about it, every THREE SECONDS someone NEEDS BLOOD.

Kari