Online Dictionaries:
An English Pronouncing Dictionary with Instant Sound
Complete website with several OUP dictionaries available online:
http://www.askoxford.com/?view=uk
Free online dictionaries from CUP
http://dictionary.cambridge.org/
Here you have Merriam-Webster Dictionary Online
http://www.merriam-webster.com/
Online English dictionary and bilingual dictionary (Spanish-English-Spanish) with phonetics, forum discussions, etc.