The number to call if you are in the US and have any question about donating blood.
According to the American Red Cross web site (http://www.redcross.org/), they need to collect 20,000 pints of blood every day. Each donation can save up to three lives.
Any healthy individual, age 17 or older, weighing at least 110 lb, is eligible to donate blood. Also, many individuals who are not 100% healthy may be eligible (e.g., I just called to ask and they told me that my diabetes does not prevent me from giving blood).