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 ~$19
HODLing!
my biggest profit for airdrop was for REBL tokens, i recieved 4000 tokens and it was almost 500 usd when they were listed on exchange, after that i didn get more then 30$ for single airdrop, now trying to do bounties,it should be better