Cool dictionary

http://www.ninjawords.com

Interesting picture dictionary and word lists

http://www.studypaws.com/elt/picture-dictionary/picture-dictionary-index.html

http://www.saberingles.com.ar/lists/index.html

Dictionaries and vocabulary

http://www.wordreference.com/
http://www.thefreedictionary.com/ (thanks, Pablo)
http://visual.merriam-webster.com/
http://dictionary.reference.com/ (thanks, Gregorio)
http://diccionario.reverso.net/ (thanks, Borja)