Gran diccionari de la Llengua Catalana (Great Dictionary of the Catalan Language).  

Catalan-Valencian-Balearic Dictionary, that is, the famous Alcover-Moll dictionary, with all the variants of the language.  

This is a free translation dictionary for English/Catalan, Catalan/English. Look at the bottom of the column on the left side, and click on "Diccionaris complets".  

Several automatic translators online, between catalan and many other languages.  

A good "Translating dictionary spanish-valencian, & valencian-spanish", on the webpage of the valencian magazine "Las Provincias". Take valencian language as synonymous with catalan!  

The last entry dictionary is based on the translating program SALT. If you want to have this program installed on your computer (free, and suitable for Windows/Linux), you can download it from here.  

Dictionary of translation English/Spanish, Spanish/English. Also monolingual in English. A good one.  

Spanish dictionary, by the “Real Academia Española”.  

Translating dictionaries, bilingual, in many pairs of languages.  

Online dictionaries from Gabinet de Terminologia in the Universitat de les Illes Balears (University of the Balearic Islands): there you will find some general dictionaries, as well as specialized ones in many areas.  

Dictionaries on almost all the subjects and in all languages, from "Universia".  

A curiosity: a dictionary in Catalan-Spanish-Latin-French-Italian.
Facsimile reproduction of the 1839 edition in Barcelona. First volume. With the sub-title: "For a society of catalan people"

Ditto, Second volume .  

And finally, a music dictionary,  

and a glossary of terms of musical notation in catalan, with equivalents in other languages.