If I have a few mins to kill during the day (usually when waiting for ETH/BTC to hit the exchanges from my wallet) Ill sign up for an airdrop or two. Most of the ones that I have either haven't hit exchanges or are worth under $1.
TRON (TRX) was the first one for me that actually did something.
500 free TRX peaked at ~$30 and is currently worth ~$19
HODLing!
I don't take part in AIRDROPS, because in my opinion, they're a waste of time. However, my friend adores participating in these airdrops and I know his most successful one is called AUTHOREON. He earned nearly $1000 on it.