The key to success is creating a viral product, and slowly leaching traffic off to your own site. Flikzor, a new video comment product that Mashable found, does both well. It is currently in limited beta with a few passes still available.
Flikzor users create a video greeting and place it via a Flash widget on any website. Visitors to the site view the video and can leave a response comment of their own, although they need to register with Flikzor to do so. See the Flikzor MySpace page to see the widget in action. After viewing a few comments you are directed to the Flikzor site to see the rest.
SnapVine and Evoca (both mentioned here) do something similar with audio-only.
For the blogger crowd, it would be very useful to have a plugin that works directly with the standard comments feature that allows users to leave text, audio, or audio/video comments at their election. If someone builds a stable plugin that does that, we’ll be integrating it across all the TechCrunch network blogs.