Free coin Airdrops.
These are coins trying to promote themselves by giving a few away in advance of launch. Individually they will likely turn out to have zero value - coins airdropped rarely take off madly in honesty.
So, when you see claims like "500usd value just for signing up" - take it with a pinch of salt, you might get 500 coins of some sort at a nominal value of 1usd each, but theres a 99,9% chance theyre not worth 1usd each when that coin launches and starts trading, rather a fraction of that. The reason for claiming them is that once in a while you might get one that doesnt tank and with no effort youve gotten a bit of value - or maybe you just sit on those 500 coins for a year or two and down the line the coin takes off.
Currently known active airdrops:
https://sphere.social/?ref_code=5309-11229c97
https://zonto.world/z43692
http://vy.tc/eGnmV24 (Openmoney lottery style airdrop)
https://actioncoin.io/?mref=hincapie
https://teleport.earth/r/61cc2844-82d5-40af-822e-44abe20d1be6
https://ico.wcex.co/?ref=apMmN4Er