What, viral like the flu?
Viral is very hard to pin down. It used to mean any bit of news or content that got widely distributed just by interested folks. Like an email forward that finds its way into your inbox -- the way those spread is a very good example of a viral phenomenon. The other term somewhat related is "word of mouth," like how a movie with no advertising gets popular.
So, good examples? Well, of comedy I would say the SNL video shorts, like "d--- in a box," which was distributed like crazy a few xmases ago (the one with justin timberlake and those other guys from the show). I think it was even a youtube hit before it aired.
(this might be uncensored)
Advertising-wise, you've got gotvmail's Gary Busey campaign
Which got blogged like it was going out of style a few months ago. That one is a weird example because the company is actually a top-tier virtual voice mail provider -- not at all who you'd expect to start a viral campaign. Arguably though it was the Buseyisms that spread the videos around the way it did.
Hopefully that sort of answers your question...