If you play League of Legends in the EU (EU West and EU Nordic & East databases), be prepared to start changing some of your account information (both in LoL and everywhere else you use the same info). It seems as if Riot Games has released an alert letting people know that their servers have been breached, leaking names, email addresses, character names, dates of birth, and security questions. Thankfully, billing info is apparently safe.
Riot states that they have hired experts to investigate the matter, and are actively making sure that this doesn't happen again. As someone who has experience in IT security, I'd advise you to immediately change your LoL password, and possibly the email linked with the account yourself (if you want to be extra safe). Additionally, if you use the same password and/or email for any other accounts (such as online retailers, banks, or even here), make sure you purge the leaked password as soon as possible.
As always, make sure you're following basic security tips to protect your information.