SlideShare will let you upload both Powerpoint, OpenOffice and PDF based presentations and share them for free. You can tag your presentation for easy searching/categorization and you can also embed them in blogs, web sites etc. It's designed along the lines of YouTube so it should be fairly easy to use and the interface should be familiar to most web users by now.
Once you've uploaded your presentation, you can view the small version or go to full-screen, which works pretty well. Looks pretty useful a service and Keynote users can use by saving their presentqtions as PDF. Maximum presentation file upload size is 30Mb
authorStream is fairly similar to SlideShare in that it allows you to upload, tag and share presentations but it only deals with PowerPoint files. However, it does have the advantage of allowing you to download a presentation as an MP4 video file and, once you have it in that format, it opens up possibilities for transferring it onto PDAs, PS3, PSP, mobile phones, DVDs, etc. The only condition of being able to download the presentation as a video is that you must set up 'Rehearsed Timings' or have recorded audio 'Narrations' in your PowerPoint file.
They also offer a free utility for Windows to batch upload PowerPoint files to authorSTREAM and also convert presentations to Flash video files. Again, presentation file size is limited to 30Mb.
Online office applications like Google Docs, Zoho Show and ThinkFree Online offer another way to upload and share presentations. All can import PowerPoint files and Zoho Show can take OpenOffice ones as well. Once imported, you can then edit the presentation directly or collaborate with others to produce a final work. Once published, the presentation can be shared with others or embedded into a web site or blog.