Investing in "real" assets is the way to go (gold, real estate, etc.). Food and drugs are always in constant demand so investing in them will continue to be profitable.
But I've had a lot of success domaining in the "temp" agency or outplacement agency industry because everyone is looking for a job. From giving interview tips to providing "blue collar" job applications (EnglishOnTheJob.com), All of my job sites are booming.
I've also had a lot of success domaining in the private edu industry (businesses like DeVry, ITT Tech, Strayer, etc.) because laid off people are going back to school to get more skills and high school grads are continuing their education to enhance existing skills.
Oh, but bankruptcy and foreclosure websites are really paying the bills these days ..They are on fire, and as long as they are getting a couple thousand unqs. per month, google adsense will pay you handsomely for each click and best of all, attorneys/lawyers will buy the sites from you for "lawyer-like" prices.
In a nutshell, a depression/recession simply provides the opportunity to invest in counter-cyclical industries (industries that boom during a recession like bankruptcy) and non-cyclical industries (industries no severely affected by booms and busts in the economy) like food and medicine.
Sometimes its hard to get quality info on where to invest because most people are cautious when it comes to telling where and how they are making money online. The "we're all fighting over the same piece of pie" kind of mindset.