I like GoldenDict. It's free to download. I have a portable version that comes with the following dictionaries:
Black's Medical Dictionary
Stedman's Medical Dictionary
Black's Law Dictionary
Black's Law Dictionary 8th Edition
La Enciclopedia Britanica Espanola Compacta
Castellano-Catalan
Catala-Castellano
Collins Dictionary of Law
Columbia Encyclopedia
Diccionario Esencial de la Lengua Espanola Vox
Diccionario Enciclopedico Vox
Routledge Business Glossary
Glosario Comercial Routledge
Routledge Technical
Diccionari Angles Catala
Espasa - Citas
Espasa - Diccionario del Origen de Palabras
Espasa - Sinonimos y Antonimos
FOLDOC
Gran Diccionario de la Lengua Espanola Larousse
Bouvier Law Dictionary
Merriam Webster Dictionary of Law
The Lectric Law Dictionary
Burton's Legal Thesaurus
Lewis's An Elementary Latin Dictionary
McGraw-Hill Dictionary of Science and Technology
DUE de Maria Moliner
Mosby Medical
Mosby Medico
Oxford Business English Dictionary
Oxford Concise
Oxford Concise
Houaiss
Michaelis Moderno Dicionario da Lingua Portuguesa
Routledge Modern Dictionary of Slang
DRAE
Diccionario Medico Mosby
VOX Medico
Concise Oxford English Dictionary
Concise Oxford Thesaurus
The Britannica Concise
Partridge Slang
Advanced English-Spanish VOX
Advanced Spanish-English VOX
Merriam-Webster Medical Desk Dictionary
Microsoft Computer Dictionary
You can probably find them online if you look around.