See this handy-dandy thing? This is a NaNo widget, which helpfully displays your word count. There are several options which you can chose here, even challenge your blogging friends to a word war in your side bar! To insert them onto your wordpress, just add a text/html widget and paste in the code they supply, name the widget NaNo or something, and save.
To put it on tumblr, paste the code into your description and wow your followers with your awesomeness!