Visual Studio Gallery
Soma has just announced, the new Visual Studio Gallery. The gallery is the central location where you can find (and advertise) Visual Studio extensions. All you need to add an entry on the gallery is a Windows Live ID, so if you have a free/open-source extension, don't hesitate to help end users locate it via the gallery.
Even if you don't have any products to advertise, it's still a great place to discover both free and paid extensions for Visual Studio. For example, check out the free StickyNotes tool that Pablo Galiano put together. It's a slick WPF-based tool window that allows you to annotate project & source files.