The New York Times have an interesting interactive diagram showing the losses that Wall St. have sustained over the last year*. As techcrunch put it:

The collapse of so many major financial institutions in the past year, and over the past few days especially, is hard to fathom in its enormity. Sometimes you need a good visual to put things in perspective. The New York Times has an interactive graphic up on its site that pretty much says it all. It shows that $4 trillion has been wiped off the total market capitalization of the U.S. stock market since last October. Of that, nearly $1 trillion is from the decline in the financial sector alone.

October 7th 2007:

September 12th 2008:

If you are interested I reccomend that you have a look at the original on the New York Times website, it has quite a bit more detail regarding the losses of individual companies (perhaps shorting JPMorgan and BoA would be a wise move?)

*Based on the Dow Jones Wilshire 5000 index.