Blizzard Entertainment is offering free character migration for players experiencing "high queue times at login," according to a recent post in the World of Warcraft forums.
The free migrations will be open "for approximately one week, or until a destination realm fills up," wrote community manager Micah "Bashiok" Whipple. "If you're looking to transfer with others we recommend setting up plans early to avoid any stragglers being left behind."
Transfers are available by logging into Battle.net, though an update to the post says that Blizzard is "aware the [Free Character Migration] page states its [sic] in maintenance and no transfers can currently take place. We are investigating."
You can learn more about free character transfers, including steps to take before migration, at the official Battle.net support site.