This is kind of a wide open category, so I'll just share a website I use more than I ever would have thought: Thesaurus.com.
While I've always enjoyed writing, I've generally depended on my own vocabulary to convey my points. One November, however, that all changed. Writing a novel in 30 days is no small feat, so I leaned on this site a lot to help me vary the verbage I used. The interesting part is that since then, I use this site for EVERYTHING. I mean emails, blogs, and even simple facebook statuses. I often find myself searching for a more accurate way to explain my situation or reveal my emotions in less general terms. Our language has an amazing ability to be very exact or generic, and the more words in your vocabulary, the better you can communicate your true thoughts and feelings, and the less chance your words will be misunderstood. Ironically, I never used an actual thesaurus much, but this online tool (sometimes the one provided with word will do, although it's a tad limited comparably) has become an integral part of my online presence.