No plugin is necessary. WordPress has support built-in. Insert this where you want the page break to be:

<!--nextpage-->

There’s a plugin out there called “Angsuman’s Multi-Page Plugin: WordPress 2.x Plugin To Create Multi-Paged Articles”, but it’s crap. All it does is add a button to your editor which pastes the above code. It didn’t even work for me, perhaps because I’m using the latest version of WordPress, 2.3.1.