Guild Wars 2

Are world transfers free? If so, is there a limit on how often they can be done?

Yes, free, every 7 days.

They will only be free until ArenaNet implements the guesting feature. No date yet on that.

At that time xfer fees will be based on the target realm population, if you were to purchase the gems with cash it is something like $20 for a high server and $6 for a low server. You could also purchase the gems to transfer with ingame gold.

Also the fee will include all of your toons, you transfer your entire account.