I'm always on the lookout for helpful online writing resources, and recently I've found a couple of good ones.
Even though I do write in English here, English is not my native language. That's what makes The Phrase Finder useful to me. It lists the meanings and origins of over 2,000 English phrases and sayings. I occasionally come across a phrase that I'm not familiar with, and it's nice to be able to look up the meaning of a phrase instead of guessing.
Another good resource I've found is Rachel McAlpine's list of articles about writing web content at Quality Web Content. There are articles on many aspects of writing in that list, and the ones I've read so far are all very good and inspiring.