Just in case you're offline sometimes and cannot use the fantastic dict.leo.org service:
Philipp Brauner made the dict.cc project available for Mac-OS (Leopard) with a really cool plugin.
It's free and well integrated into the standard dictionay application.
(You Mac users did know that you can translate any (highlighted) word from any Cocoa application by hitting Ctrl-Cmd-D, did you?)
