Merriam Webster Visual Dictionary Online - Perhaps too advanced for early beginners but useful for more advanced Learners
Vidtionary - A video dictionary. Useful for words that require more complex definitions. Brief videos explain the words.
7ESL: 7 Steps to Learn English - Comprehensive vocabulary and grammar resource.
Dave's ESL Cafe - Online Grammar Lessons that Tutors and Learners can utilize.
Little Explorers - A picture dictionary oriented towards children. However, for basic learners this may be quite useful. Advanced feature require a fee.
NASA's Picture Dictionary - Space and aeronautics terms.
Rhyme Zone - Find rhymes, synonyms, adjectives, and more! Poems anyone?