PowerPoint+Gallaries

http://www.authorstream.com/ -- a wide collection of PowerPoint presentations. Lots of student work, but some other collections, too. Good search engine.

http://www.slideshare.net/ -- the most widely-used PPT sharing site. Lots of presentations from businesses; some student work, too.

Some others (thanks to Larry Ferlazzo)

[|Authorstream]

[|Slideboom]

[|280 Slides]

[|My Brainshark]

[|PowerShow]

[|Slideserve]

[|MyPlick]

[|SlideSix]