OSXOpenDic is a free open source dictionary engine (written for Mac OS X) developed using SWT. There is an English-Persian DB file available for download which is non-free but distributable. (Use the import tool to import the xml file into the database).
InstantTranslate makes it really easy to translate a selection of text in the application you are working in from one language to another language.
One World Dictionary by Ascendo is a leading multilingual reference tool for Apple desktops.
