Music service Last.fm was hacked for a total of 43,570,999 users. Each record of the stolen data contains a username, email address, password, join date, and some other internal data. The company currently investigating the leak and asking all users to change their passwords for the service immediately.
The passwords were stored using unsalted "out of style" MD5 hashing not plaintext. Hashing is a method for encrypting data, but some methods are far superior to others. Salting is a practice of adding a random string of numbers to the hash for each individual password, making them more secure and decreasing the likelihood that they will be cracked if the passwords are ever leaked online.
You may search for yourself any of the leaked databases by visiting the homepage of the leaker. If your personal information appears in one copy of this database, or in any other leaked database that they possess, you may remove yourself for free.
Funfact: The most popular password from the Last.fm hacked database was 123456.