Seriously noone has mentioned Unique Article Wizard? Check it out it blows the other 30 or so article writers I've tried out of the water. It contains a synonym database updated by users so the synonyms actually make sense and are available for entire phrases as well. There is no magic pill that lets you click a button and bam you have a new unique article. That doesn't and will never exist so stop lookin if you are(or squander your hard earned $ on failware)!
Spin Rewriter integrates with almost any SEO software. This allows you to post high quality articles quickly and easily. The only drawback is that you only get something like 300 API requests every 24 hours. You can quickly burn through this if you’re doing multiple campaigns. The developer does not currently offer a plan for heavy API users but maybe they will in the future.