Word Counter is an easy to use online tool for counting characters, words, sentences, paragraphs and pages in real time.
You can also use it to track your text length against common web standards like Twitter's tweet character count (280), Google's meta description (160), and Facebook's average post display length (~250).
Wordcounter shows a real-time word count as you type in any Google Docs document. You can toggle it off and on by clicking on the icon in the top right corner of your browser.
It also adds milestone markers every 500 words. Motivating!