- WordReference.com: WordReference is my first choice in Spanish, and it's often the only choice I ever need. Not only does it provide you free access to a few different traditional (read: dead-tree) dictionaries whenever you look up a word, but it also has its own content.
If none of those get you the definition, then it's time to turn to WordReference.com's forums. With hundreds of thousands of entries in the forums, I've almost always found that whatever I'm looking for that's not already in the dictionary is already in the forums. And, when that fails, posting something in the forum will get you a quick response. For example, I put up this post when I was trying to figure out how to say "I'll spare you the details" in Spanish (f.y.i., it's "Te ahorraré los detalles"). I got the answers I needed in a minute. That's right; one, single minute.
- Reverso: Reverso doesn't have quite the depth of WordReference.com's content, but it is a solid dictionary and once in a while I manage to find a phrase there that I couldn't find in WordReference.com. Given the breadth of WordReference.com and its powerful forums, I rarely use this, but it does provide a good back-up.
- SpanishDict.com: SpanishDict.com takes third place here because it doesn't seem to have quite the breadth of the first two. It does have concise definitions, so it might be a good thing to use if you're just starting out and don't need the level of detail found in WordReference.com. Like WordReference.com, it also has forums used in a similar manner, but they're no where near as developed as WordReference.com's.
You might note that I didn't mention example sentences or the ability to listen to words being pronounced at all above. That's because, unlike the selections in Japanese and Chinese, there are no dictionaries that I'm aware of that implement these features in Spanish. Although you can find pretty much any word you like in the above dictionaries, you're often on your own to find a way to hear the pronunciations (here's one way to do that) and to find example sentences. There are of course some example sentences in the dictionaries above, but there's definitely no systematic sentence collection from net sources like that found in the Japanese-English dictionary ALC.
As always, let's hear what you've got to say in the comments below—especially if you can point me to an even better dictionary that I missed!