I joined the Bone Marrow registry after reading a reddit post a few months ago, and yesterday I donated to (hopefully) save an 18 year old boy's life. AMA.
It was pretty cool and surprisingly easy. After I joined, they immediately called and said that I was a match and needed right away. All they can tell me is that it was for a 18 year old male from the United States. After a year, we can both agree to learn about one another, but you can keep it anonymous if you like.
The interesting part is that they can do directly from your blood stream in some cases (like mine). They gave me a drug each day for 5 days leading up to the procedure which caused my marrow to create excess stem cells which leak into my blood stream. When I have enough, they just run it through a machine and back into me over about 4 hours. Super easy and they treat you like royalty when you do it. Plus I got free candy.
TL;DR; It's easy and you can save someone's life.