This looked neat, so here you go.

For the nerd/doomsayer in all of us, I present to you ten scientifically feasible ways that the Earth could meet its doom: http://www.livescience.com/technology/10ways_destroyearth.html

I actually had to look one of them up because I had never heard of it before. That's always odd...

But then there's always the fun alien invasion (sorry, watching ID4). But that list actually encompasses ways to obliterate the Earth. Not just whipe out the species or something. So if you're looking for ways to destroy those pesky Earth-like planets (in the means of smashing it with the ol' cosmic hammer) then this is your list.

Other than that I actually have a myriad of scientifically feasible/natural ways to destroy all life as we know it on the Earth.

And if you liked that one, you'll possitively love this one: http://ned.ucam.org/~sdh31/misc/destroy.html#methods

It follows a similar method but goes a great deal overboard in also including some of the fictional/improbable ways to do it and then bringing up ways NOT to do it. They post the site with the aims to actually accomplish the distruction of the Earth. Some of us actually see that as a waste and wish to dominate it instead, but oh well...