Lingoes Portable is both a free portable dictionary and portable text translation software. It features a complete dictionary and can translate text and pronunciation of words in over eighty different languages.
Yadabyte Dictionary is an open source, free portable dictionary that is very fast, efficient and most importantly, super easy to use. Like other Yadabyte products, it features only the navigation features you need without the clutter.
Magical Jelly Bean offers an Open Source dictionary that can be stored on a USB device. The portable dictionary does need to connect to a dictionary server to retrieve definitions from the database on the server.