SlideShare is a free web service that allows users to upload, tag, embed, comment on, and share PowerPoint, OpenOffice, Keynote, and PDF presentations. While other companies like Scribd and Google Docs offer some of the same features, with SlideShare you can easily create "slidecasts," a multimedia format that synchronizes your presentation with an mp3 file hosted on another server, using the service's straightforward web-based slide sync interface.

Below is an example of a Slidecast: