One of the comments about YouTube in the media flurry is that it was designed to be very easy to use. One aspect of this is the convergence on Flash as a way of presenting/delivering resources. This does away with the need to struggle with multiple players, and supports the easy linking and embedding which is promoted by YouTube. The emphasis is on ease of use and reuse.
Via eFoundations, I came across SlideShare, a presentation-sharing service. It has a strong family resemblance to YouTube, including the emphasis on ease of linking from and embedding in blogs and other sites. Tagging and comments are supported as one would expect. I uploaded one presentation which worked very smoothly. It does not currently support slide builds, and I could not immediately see how to link to an individual slide.
This is early days and you need to register to use it. This is a potentially useful service: it will be interesting to see if it lasts.
Update: As Paul and Andy point out, it is possible to link to individual slides. This is by pasting the URL from the box on the side, rather than from the toolbar (where you might expect it). That said, it appears that all that you need do is add the slide number to the base URL for direct access to a particular slide.