Who Is "Mystery Babylon"?

There has been a lot of discussion of eschatology over the last one hundred and fifty years, but in this day and age, as we are entering into the "beginning of birth pangs" (Matthew 24:8) there is a lot more discussion about it, as different sects of the Church believe different things about Jesus' Second Coming.

Arguably the most confusing part of the end-times is that of the identity of Babylon. Who and what is it? I would love to hear your opinions and reasoning (I will also post mine and responses).

Please be sure to cite your sources and be courteous insofar as having credible ones (no youtube videos or wikipedia entries, please!). It is preferable that you use Scripture as much as possible, actually, unless you are projecting an interpretation (even then, please have some sort of inductive support).

Have fun :)