Is there an out-of-the-box solution or workaround to create a drop-down menu for the Top Navigation link bar (horizontal navigation) in SharePoint 2010? Or is there a suitable Bamboo custom web part for it?
You can create a drop-down kind of a menu for the horizontal navigation Out of the box. If you have any subsites/pages under your site collection/subsite that you want to show in a drop-down manner, you can use the "Navigation" option in the "Site settings" to configure the horizontal navigation bar. For this you need to have publishing features enabled in your SharePoint site. Once you have enabled this you can see the "Navigation" link in Site settings page under "Look and feel". Here is a detailed post that will help you create one.