There’s a new online website from Purdue University that allows you to calculate and simulate the potential damage of a comet or asteroid on Earth. The Impact: Earth site allows you to enter in different parameters such as mass, velocity and angle, and then assesses the damage on impact. The site returns such metrics as atmospheric blast wave, ground shaking, size of tsunami generated, fireball expansion, distribution of debris and size of the crater produced.
The site provides an entertaining science-based exploration of impacts, with a degree of realism good enough for use by the Department of Homeland Security in their own simulation scenarios. There are also details here on famous craters, with information on the estimated size and density of the projectile.