If you are playing World of Warcraft, a classic Blizzard game or planning on buying any of the new Blizzard games (Diablo 3, Starcraft 2). Then make sure you are registered on Battle.net.
You can “merge” your World of Warcraft accounts with it as well. Unfortunatly it is restricted to American World of Warcraft accounts. European are “Coming Soon! The World of Warcraft account merge process is not yet available in your region”.