studyspanish.com is the best overall web resource for learning Spanish. They have lots and lots of free stuff (literally thousands of pages!) and they also offer an audio CD course that is based on the well-proven Foreign Service Institute course. Highly recommended!
medicalspanish.com is a new and developing site that is geared especially for health care workers. This looks like it is going to be one fantastic resource, with nursing CE to be available in June. Already available is medical Spanish vocabulary, general Spanish vocabulary, patient teaching aids, etc. Watch this one!
spanishvocabulary.com is a links-only site with links to some of the best Spanish vocabulary resources on the Web.
spanishverbs.com is a another links site that focusses on resources for learning Spanish verbs.
cyberspanish.com content on this page is constantly changing!