I recently saw two different viral marketing videos with a twist: they incorporate user content to pull in the viewer. Stepbrothers: Duel is an ad for an upcoming movie comedy, and uses your name, photo, and phone number (!) to create a video of Will Ferrel and John C. Reilly fighting over which of them is your best friend.
Election '08 prank is purportedly a news story about a groundswell presidential candidate. Scroll to the bottom of the page, enter your name, click Update, and play the video. They insert your name as text elements in a number of places. If you listen carefully, you'll notice they cleverly avoid any gender-specific pronouns, unlike the Stepbrothers video, which will only work for a male:
This is clever technology, and will keep viral marketing campaigns alive (I just spread two of them), that is, until it gets old.