For more than 100 years, people have been throwing trinkets and novelties into the crowds attending Mardi Gras celebrations in New Orleans, Louisiana. Around 50 years ago, designers came up with an interesting souvenir that was lightweight and could be inscribed with the theme and logo of each particular carnival group (also known as a 'krewe').
These tokens - also known as 'doubloons' - quickly became popular among collectors and the Mardi Gras crowds (the crowds especially like them because they are made of plastic, aluminum or other lightweight metal and don't cause injury when they're being thrown from a parade float).
The designs change annually to correspond with each year's parade topic. The doubloons are typically larger than a silver dollar. While many of them have the original metal color that looks grey/silver, quite a few of them are also colorized - blue, yellow, green, red, and purple are all popular doubloon colors. Values range from a few cents apiece for most modern tokens to around $10 for some (more desirable) earlier pieces. The more desirable tokens are generally from the 1960s and early 1970s, not made of plastic, and also bear the logo of a popular or well-known krewe.
There are other doubloons that have been manufactured strictly for collectors. These ones are made of heavier metals (you can't throw these ones into a crowd!) and most of them are either sterling silver or .999 fine silver. There are bronze issues as well. Bronze issues are typically worth about $1 apiece and the silver issues are worth about $25 apiece. As with the throwable versions, the more desirable silver and bronze doubloons with earlier dates and well-known krewes on them can have an added premium - generally not more than $5 or $10 though.
If you're interested in collecting these tokens, they are readily available online and many coin shops have them as well. The Mardi Gras celebration takes place every year in New Orleans on the day before Ash Wednesday (first day of Lent), but the parades and celebrations begin a few weeks before that. If you're in the area you can go catch some doubloons for free!