The menu bar is a quintessential piece of the Mac "experience." On the left, it shows all of the menu items for the currently active application; on the right, it shows icons from various menu bar application programs: the date/time, WiFi status, MobileMe sync status, and many, many more.
The whole idea is to make the menu items easy to find and easy to "hit" with a mouse, by being connected to the top of the screen (Fitts's law and all that). But what if you have two monitors? Suddenly the menu bar might be all the way over on the other side. Yes, I realize that complaining about using a Mac with two monitors might be the quintessential "first-world problem," but the more minor irritants you can remove from life, the better.
SecondBar is an app which will extend your menu bar to a second monitor. I've been using it for a while and it works pretty well for what the author describes as an "alpha" build.