Credit default swap
This is because the way lots of companies speculated was by hedging on credit default swaps. They bought credit default swaps for a company, and then sold credit default swaps for the same company when the CDSes got more expensive. For example, if you bought some Lehman Brothers CDSes from AIG where you had to pay $500,000, and you sold the same number of CDSes on Lehman a year later for $600,000, you made $100,000 profit. If Lehman defaulted, you're supposed to pay the people you sold the CDSes to, but that should be OK because now AIG is supposed to pay you for the CDSes you bought.