Lately there have been a number of scam schemes in chat that could affect you In Real Life. Please read carefully since this can save you a lot of trouble!

Hacked Accounts

There are players offering free "Premium" Accounts. They can be used to login Premium, as well as Cracked Servers.
Mojang's policy is to ban such users. When this happens you will lose the abillity to login Premium Servers.
The worst part is your IP gets banned from the Premium Mojang Launcher. This means no more Premium Minecraft for you even if you buy the game.
Do not accept free "Premium" accounts from players. Ask yourself are you going to give out accounts you bought with your money to strangers?

Spy Links

Normal players can find your IP if you click on a link they give you.
Knowing your IP they can DDOS you, find your personal details or worse (bad things can happen)
The spy link is normally disguised as a shortened URL like:
  • TinyURL
You never know what is hidden behind a shortened URL so NEVER click them unless you trust those people.
If you do click on a spy link it will get your IP and redirect you to normal content like a picture. So you won't even realise you were fooled.
If you want to find out whats hidden behind a shortened URL you can use an URL Expander Tool like

Safe links to click are Youtube, Dropbox, Imgur, TinyPIC (not TinyURL), Gyazo, and other familiar sites.

"Can you come to my server?"

By joining a server you allow certain staff (with admin access) to see information about you like your IP.
They can also view any password you register with. Do not use one password for all your servers!

Joining unknown servers offered by strangers can be dangerous. Before you join make sure there are other people online. If there is noone of this server it's either a trap or not worth your time.