Marketica v2.0 allows you to turn your store into a vendor marketplace. You can assign existing products to vendors, or they can add and edit their own ones. Every vendor will have a dedicated vendor page on your site where they will be able to list all of their products, but they will also show up in your normal shop pages so you can sell your own products alongside those of your vendors.
Demo: http://themeforest.net/item/marketica-marketplace-wordpress-theme/8988002