Online Spanish Dictionary:
SpanishDict.com Online Spanish dictionary with audio pronunciation
FreeDict.com Online Spanish Dictionary with 20000 entries
Wordrefernce.com Free online Spanish translation
Spanish Oxford dictionary on CDROM
(Great search features, very complete and useful)
details and price at Amazon.com
Pocket dictionaries for travellers:
Pocket Spanish Dictionary by Langenscheidt
details and price at Amazon.com
VOX Compact Spanish and English Dictionary
details and price at Amazon.com
Webster's New World Pocket Spanish Dictionary
details and price at Amazon.com
For your reference at home & office:
Webster's New World Spanish Dictionary
details and price at Amazon.com
The Oxford Spanish Dictionary
details and price at Amazon.com
Cassell's Spanish-English, English-Spanish Dictionary
details and price at Amazon.com
The University of Chicago Spanish Dictionary
details and price at Amazon.com
Visual Dictionaries:
The MacMillan Visual Dictionary Español/Inglés
details and price at Amazon.com
The New Oxford Picture Dictionary
details and price at Amazon.com
Other recommended resource:
501 Spanish Verbs
details and price at Amazon.com
Free Online Spanish Translation Tool.