Wow, we haven't had one of these for a while. Starting yesterday (oops), players on a few high-population European PvP servers will be able to transfer to some of the lower-pop ones for free:
Due to population differences between some of the highest and lowest populated PvP realms, the following Free Character Migration will be available from October 24th to 30th:
From: Crushridge, Frostmane, Grim Batol, Kazzak, Outland
To: Boulderfist, Jaedenar, Kor'gall
Please note that the migration may be closed at any time should a destination realm's population rise too high. (Thundgot)
Edit: Apparently the following additional EU migrations are also active until the 30th:
To: Uldum, Tyrande
As the post says, you should take advantage of this ASAP if you want to move, because they do occasionally close these early.
Are you on any of these servers? If you're on the "from" realms, do you agree that overpopulation is a problem, and if so, are you going to move? If you're on the "to" realms, do you think your realm needs more players?