Terror Attacks in the US - Visualized

The Guardian parses through the Global Terrorism Database, and charts the results above.
Some important caveats - these are

• a count of attacks, not of impact
•  not a count of fatalities

The second point is particularly relevant - you have some attacks (9/11) with large fatalities.
All that said, it is important to note that, in the end, attacks are quite rare - it is worth turning it over to Bruce Schneier here

As the details about the bombings in Boston unfold, it'd be easy to be scared. It'd be easy to feel powerless and demand that our elected leaders do something -- anything -- to keep us safe.

It'd be easy, but it'd be wrong. We need to be angry and empathize with the victims without being scared. Our fears would play right into the perpetrators' hands -- and magnify the power of their victory for whichever goals whatever group behind this, still to be uncovered, has. We don't have to be scared, and we're not powerless. We actually have all the power here, and there's one thing we can do to render terrorism ineffective: Refuse to be terrorized. 

It's hard to do, because terrorism is designed precisely to scare people -- far out of proportion to its actual danger. A huge amount of research on fear and the brain teaches us that we exaggerate threats that are rare, spectacular, immediate, random -- in this case involving an innocent child -- senseless, horrific and graphic. Terrorism pushes all of our fear buttons, really hard, and we overreact. 

But our brains are fooling us. Even though this will be in the news for weeks, we should recognize this for what it is: a rare event. That's the very definition of news: something that is unusual -- in this case, something that almost never happens