Lexical Database

WordNet® is a large lexical database of English. Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. Synsets are interlinked by means of conceptual-semantic and lexical relations. The resulting network of meaningfully related words and concepts can be navigated with the browser. WordNet is also freely and publicly available for download.
Urban Dictionary (UD). UD is a popular online slang dictionary with definitions written and voted on by users.
Natural Language Processing - (Opinion mining | Sentiment (Analyze|classification)) - Emotion: Opinion Mining, Sentiment Analysis, and Opinion Spam Detection with an Opinion Lexicon (or Sentiment Lexicon). A list of positive and negative opinion words or sentiment words for English.
The Harvard General Inquirer is a lexicon attaching syntactic, semantic, and pragmatic information to part-of-speech tagged words.