Because of the financial situation in Argentina you can get a better exchange rate on the street than the official rate issued by the government. This so called blue market operates in a kind of grey area. Officially it isn't allowed but the police turns a blind eye to it. We've seen arbolitos (literally little trees) shouting for cambio (exchange) in front of policemen.
There are different websites and twitter accounts which inform about the daily blue rate, even local newspapers print it next to the official one!
All this started after the peso lost a lot of its value as a consequence of the poor economy. People being afraid of loosing their savings started frantically buying USD. This led the government to rationing the amount of dollars a person can obtain to prevent the drain of money "out of the country".
Lately the government tried to defuse the situation by allowing argentinians to have a bank account in dollars. For this account they can buy certain amount of dollars at the official rate but have to pay a 30% tax the moment they withdraw it (other people told me this tax expires after one year).
After our debacle in Venezuela we came prepared to Argentina. We had a fair amount of peso chileno and dollars to exchange.
As the golden times (at least for us foreigners where we would get twice the amount on the blue market) are over we ended up exchanging our money the tourist way on Calle Florida.
What we didn't know before is that you get better rates for higher denominations. Which means bills of 50 and 100 USD would fetch close to the price advertised. Everywhere else in the world it is a hassle to cash in bills higher than 20$ so we didn't bring any. We still got 10.50 per dollar when the price on twitter was at 10.90 and the official rate was at 7.90.
There are some misprinted new 100 Peso bills in circulation which certain places don't accept, so make sure you get old ones or know how to spot them!
PRO TIP: Bring big dollar bills to Argentina!