1. A method of creating a time-based media composition, the method comprising: using a computer-based media composition system connected to a wide-area network to search for secondary material available from secondary material providers connected to the wide-area network; selecting an item of secondary material located during the searching step; receiving over the wide area network descriptive metadata pertaining to the selected secondary material item; inserting at a temporal location within the time-based media composition a representation of the selected secondary material item, wherein the time-based media composition includes primary media; and outputting a rendered portion of the time-based media composition spanning the temporal location, the portion including a portion of the primary media and a portion of the secondary material item, wherein outputting the secondary material portion involves outputting an evaluation version of the secondary material item that is streamed to the media composition system over the wide area network.