The New York Times has a great interactive graph, which compares the current stock market slide (Dow Jones) with that of other slides.
It shows that this market fall is roughly about the same as that which occurred in 1974, but it has happened in about half the time.
The market has also fallen about the same amount as occurred from February 2000 to October 2002. So what took nearly 3 years (due to the dot.com crash) has happened in about one year. The depth of the falls that aren't so much the problem (though of course they are a big part), the real problem is the speed.
The market still has a ways to go before we get to Great Depression levels, so here's hoping things improve well before then.