The differences between dictionaries with simplified and traditional characters:
Who should choose dictionaries with traditional characters?
Note: In practice, it is easier to pick up simplified characters after learning the traditional forms, rather than the contrary.
Oxford Concise English-Chinese Chinese-English Dictionary
Far East pinyin Chinese-English dictionary
Mathews' Chinese-English dictionary : a Chinese-English dictionary
Chinese-Chinese dictionaries are incredibly useful for advanced Chinese learners.
Chinese Simplified Traditional Dictionary (汉字简化字与繁体字对照字典)
Xinhua Character Dictionary (新华字典)
Contemporary Chinese Dictionary (现代汉语词典)
There are numerous dictionaries that deal with specialized subjects, and examples at UNC include:
Multilingual
Chinese Dictionaries
Subject Dictionaries