If you or a friend have been looking to try Guild Wars 2, are still unsure about buying the game (you should), or just want to kill some time, ArenaNet is partnering with sites to hand out thousands of codes for a trial taking place over the April 19th weekend. Note that different sites are doing give-aways at different times.
Here's a list of places you can get them:
EDIT: Updated the participants. Evidently, GameSpot, Twitch.TV, and PC Gamer will also have keys at some point.
Also, if you participated in the last free weekend (the one in November), that account will be registered for this weekend as well.