Deleted "Application-Places-System" on Top Panel in error

Right-click on the top panel (i.e. the bar at the top of the screen that used to contain your Applications, Places, and System menus). Click "Add to Panel...", select "Menu Bar" in the list, and click Add. Then click Close.

Now, if the menu you bar you just re-added isn't all the way to the left of the top panel (and you want it to be), right-click on it, click Move, move your mouse pointer left until it shows the menu bar where it's supposed to be, and left-click.