I will be giving a presentation in a few weeks and went to two of my favorite sites that have emerged as vibrant repositories for all sorts of presentations.
I use Scribd and Slideshare as a portal to see what people have posted on the topic that I will be discussing. Great homework for presentations, and also a good way to keep up on happenings on conferences (where the organizer does not post for open viewing). For example, if you go to Slideshare, a search on the SXSW conference returns these six pages of presentations. Not all relative to the topic, but enough are to satisfy my direct needs, as well as spurring a search journey into other areas.
For example check out Tim O'Reilly's 2008 CIO exchange presentation here. It could use a bit of editing, but the content, concentrating on markets where technology seems to be in the early stages of being potentially disruptive is really strong.