Resources grouped according to your level:

Resources for elementary students:
Elementary and Upper-Elementary levels

Resources for intermediate students:
Lower and Upper Intermediate levels

Resources for advanced students:
Advanced, Superior and Proficiency levels

Quick links:

Dictionaries:
WordReference Spanish dictionary
Spanish dictionary with audio
Collins (Español-Inglés/English-Spanish)
Real Academia Española Diccionario de la Lengua Española

Verbs:
WordReference verb conjugator
Spaleon verb conjugation trainer
Regular verbs

Newly added audio and video sites:
Notes in Spanish - podcast audio conversations from Spain for all levels - excellent!
Destinos - an instructional telenovela (soap opera!) - surprisingly good!
spanish/audio1/listeningindex - a very comprehensive site with audio, exercises, vocab lists...

A glossary of grammatical terms:
Useful!