Mythic just announced free 10-day trials for Warhammer Online through FilePlanet, which is available to Australia, Japan, New Zealand, Singapore, and U.S. residents only. Up until now, the only way to try WAR for free was if you received an invitation from Mythic's Recruit-a-Friend program. For a limited time, anyone to takes the free trial through FilePlanet will also receive $5.00 off the full purchase on Direct2Drive.
You can download the trial here. The trial will expire 10 days after you activate your account, even if you don't log in. Trial accounts are restricted to tier one and capital city zones. You may purchase items from the auction house and receive in-game mail but you can not sell items on the AH or send in-game mail (to prevent spam/bot accounts). Trial characters can only be created on designated servers. Check the official free trial FAQ for more details.
It is unknown when or if the 10-day trial offer will expire. The ideal time try WAR for the first time is probably in early March once they implement the 1.2 patch. Patch 1.2 will feature the Bitter Rivals live event (many interesting tasks, a new scenario, and will usher in the Choppa and Slayer careers), fix hundreds upon hundreds of bugs, and aim to balance almost all the game's careers.