If you're Indian, and Blood Type O, you should know about Bombay Blood Type
I got into a discussion recently with my favorite medical student about something called the "Bombay" bood type. It's very rare, unless you're from India. Out there, it occurs 1 in 7,600 people. Most people who have it wouldn't even know that they do; on most tests, they will show up as type O.
Most of the time, it doesn't matter at all. They can donate blood just like the O blood group. The hitch is when they need a blood transfusion. They can only receive blood from someone who has the Bombay blood type, and it's very rare.
There may be issues around bearing children, too, for women who have the Bombay blood type, issues resembling the Rhesus negative problem (which is much more common).
This entry is mainly to get the word out. There's a wikipedia article about it, too. If you are from India, and have the O blood type, consider getting tested for the Bombay phenotype. If you have it, you may need to know.
Someone's established a website to link up people with rare blood. [ETA, May 2010: Unfortunately, it looks like the website is dead. I've removed the link.]