Busy again, but trying to catch up - fortunately, the next few things are things I've already been using.
I found out about the online slideshare sites through 23 Things, and they fill one fantastic niche - you know how whenever you go to do a presentation, something always goes wrong? Having the presentation on an external site helps if something goes wrong. In fact, I uploaded the presentation from mid-February when I finished it, just in case something went wrong with the emailed version or my USB drive. You can view it here.
So, that's already one fantastic use. The peace of mind alone is worth quite a bit to me. But there are other benefits - sharing a presentation in progress without sending huge files back and forth, for example.
I found Slideshare relatively easy to use, though I hd some uploading issues (the default method didn't work for me, but using the manual upload did. I consider that a quirk of interacting systems, mostly: I was trying from my work machine which runs Windows XP and Office 2007.) Actually viewing the slides was easy, though - and the friend who arranged my presentation had an easy time viewing them in advance, too.