As announced by Blizzard poster "Vaneras", crowded European WoW servers will get a limited free transfer opportunity to the less populated servers between January 16th and 22nd:

Due to population differences between some of the highest and lowest populated PvP realms, the following Free Character Migration will be available January 16th to 22nd:

From: Al'Akir, Burning Legion, Stormscale, Magtheridon
To: Daggerspine, Spinebreaker, Xavius

Please note that the migration may be closed at any time should a destination realm's population rise too high.

If you've been contemplating moving, then hurry up before all the slots are filled!