While in Brazil in 2000, I got this bill in change at a store. It's a 20 Reais note, produced especially for the 500th anniversary of the country (counting from Pedro Álvares Cabral's discovery on April 22nd, 1500.)
What's interesting about it is something that's difficult to sense without running your finger over the surface -- it's printed on polymer plastic instead of cloth paper. The red circle on the left-hand side is actually transparent plastic. When you fold the bill over on itself, the numerals "10" appear when viewed through the red filter.
Here is an image of the back of the bill. Note the multi-racial faces on it -- everything from white to black to indigenous. For more detailed info, see this page on the website of the Banco Central do Brasil, or this page in English.
Although 10 Reais is about $3.70 at today's exchange rate, the bill itself is allegedly worth $12-$15 as a souvenir. That trip to Rio is paying for itself already.
{Side note: the currency is Brazil is one real, plural reais. Initial 'r' goes to an h sound in most Brazilian dialects of Portuguese, and terminal 'l' causes the lips to be rounded into almost a w sound.}