Okay, the details are still murky, but the explosions everyone was talking about turned out to be IED's (yes, IED's) and there were others that had not gone off. I'm hearing other stuff but I'm holding my peace as there's a lot of confusion.
If you have a loved one who was running in the marathon, you can check their status here. If you are safe and well and (and known to be at the marathon), please register at the Red Cross's Safe and Well page. If you are looking for a loved one who was at the marathon, you can check the Red Cross Safe and Well listings to see if they've registered.
Do not try to give blood if you're in Boston today. The hospitals have enough right now and they're busy. (Apparently, people in the area want to do something, which is understandable.) Massachusetts General Hospital is taking walk-in blood donations at 7 AM tomorrow. They will need more blood donations so if you can do it it would be much appreciated.
Also, I want to stress: We do not know who was behind this. Please do not go making any assumptions about who has done this. And once we know, please do not go around making assumptions about people who share the perp's/perps' ethnicity or background. I have friends of all ethnicities and religions; I do not yet know if the Boston based friends were in the area of the bombing. They will suffer and bleed and die just like anyone else.