Well drugs. As much as people want to dismiss drug culture because it's illegal (not necessarily meaning that it's immoral). Even if you dismiss the use case of drugs, it's hard to deny how successful these sites are at pioneering nearly p2p marketplaces compared to something like Amazon.

There is also the case of having a safe storage of funds. The seizures of bank accounts in Greece and the actions some governments are taking to stop capital flight speak to the use of bitcoin as a way of storing value.

Gambling is another huge use case. Anyone who has a lick of experience dealing with deposit/withdrawal from gambling services know how inconvenient it is to actually move money around.

Online sales are another use case. It's far easier to verify buyer information and funds. You also save the 10%+ in fees that you would pay using a service like ebay.