Twitter's Boostrap framework is great for making affixed menus that stay with you as you scroll down the page. I recently needed to do this and got it working pretty well. It's also responsive, so the mobile dropdown menu scrolls with you as well. This is my favorite navigation design pattern. I've taken the liberty of making an example that has a fade-in effect so it isn't so jarring when the menu affixes.
Here's the JSFiddle of it in action: