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All proof-of-stake (PoS) coins earn interest, for example Reddcoin, Black coin, and Peercoin. For those you leave them in your unlocked wallet, and they give you more coins (depending on how much you start out with, and what the interest rate is for each coin). There are a bunch of coins that have higher interest rates then the ones I mentioned too.

But SBD would be cool because I use Steemit every day, and it's convenient that it sits on Steemit so I don't have to download yet another wallet. Oh well, maybe one day the interest rate will go above 0...i just don't know what will cause that !

First i've ever heard of a cryptocurrency earning interest. Good to know and would be great if that was applied here.

When you say " ... For those you leave them in your unlocked wallet, and they give you more coins "

What kind of wallet does this need to be? Can it be Poloniex?

It needs to be the coin's wallet that you download and run on your computer. Leaving your coins on Poloniex, or any exchange, won't earn any interest for you.

Ok thank you for the clarity!

Also worth mentioning the obvious: it's not safe to leave your coins on an exchange, you don't own the private keys so you don't actually own them.