DBPedia : Entities and relationships are automatically extracted from wikipedia.
Wordnet: Lexical database of english language. Groups english words as synsets and provides various relationships between words in a synset. It is a knowledge base that tracks specific kinds of relationships like synonym, antonym, hyponymy and so on. http://wordnetcode.princeton.edu/5papers.pdf
Yago : Also extracts knowledge from wikipedia. But also focuses on the spatial and temporal dimensions of facts
Freebase : community curated, beyond wikipedia, rich in media related data music/movies it has been integrated with wikidata
Wikidata : community curated knowledge base . Freebase has been integrated with it.