Probably the best (free) calendar (and the one i primarily use is Forex factory).
http://www.forexfactory.com/calendar.php
Benefits of the above calandar are that very rarly does the site become overburdened/flooded when a big announcement is relesed like some other forex sites, the announcements are timed very well and never late and well i havent found a better calendar yet and have been trading forex for many years!
