AIG
Here is an article on AIG by the New York Times' Joe Nocera. The article clearly explains AIG's credit default business, why the US government can't let AIG fail, and AIG's greed and stupidity.

Here is a standout point:

Why would Wall Street and the banks go for this? Because it shifted the risk of default from themselves to A.I.G., and the AAA rating made the securities much easier to market. What was in it for A.I.G.? Lucrative fees, naturally. But it also saw the fees as risk-free money; surely it would never have to actually pay up. Like everyone else on Wall Street, A.I.G. operated on the belief that the underlying assets — housing — could only go up in price.

That foolhardy belief, in turn, led A.I.G. to commit several other stupid mistakes. When a company insures against, say, floods or earthquakes, it has to put money in reserve in case a flood happens. That’s why, as a rule, insurance companies are usually overcapitalized, with low debt ratios. But because credit-default swaps were not regulated, and were not even categorized as a traditional insurance product, A.I.G. didn’t have to put anything aside for losses. And it didn’t. Its leverage was more akin to an investment bank than an insurance company. So when housing prices started falling, and losses started piling up, it had no way to pay them off. Not understanding the real risk, the company grievously mispriced it.

And here is one reason why the money market system almost collasped last fall:

There’s more, believe it or not. A.I.G. sold something called 2a-7 puts, which allowed money market funds to invest in risky bonds even though they are supposed to be holding only the safest commercial paper. How could they do this? A.I.G. agreed to buy back the bonds if they went bad. (Incredibly, the Securities and Exchange Commission went along with this.) A.I.G. had a securities lending program, in which it would lend securities to investors, like short-sellers, in return for cash collateral. What did it do with the money it received? Incredibly, it bought mortgage-backed securities. When the firms wanted their collateral back, it had sunk in value, thanks to A.I.G.’s foolish investment strategy. The practice has cost A.I.G. — oops, I mean American taxpayers — billions.

Here is why the government can't allow AIG to fail:

Here’s what is most infuriating: Here we are now, fully aware of how these scams worked. Yet for all practical purposes, the government has to keep them going. Indeed, that may be the single most important reason it can’t let A.I.G. fail. If the company defaulted, hundreds of billions of dollars’ worth of credit-default swaps would “blow up,” and all those European banks whose toxic assets are supposedly insured by A.I.G. would suddenly be sitting on immense losses. Their already shaky capital structures would be destroyed. A.I.G. helped create the illusion of regulatory capital with its swaps, and now the government has to actually back up those contracts with taxpayer money to keep the banks from collapsing. It would be funny if it weren’t so awful.

I worked for a company that was bought by AIG and have noted before my dealings with the "insurance" people from AIG's headquarters. These people did not impress me with their intelligence. AIG's implosion did not surprise me, but the extent of its stupidity and how other Wall Street firms exploited its stupidity is amazing.