What is it?
Slideshare.net is a mixture of a content hosting site (people upload their presentations there as a way of storing them online), a content sharing site (people will give the link to the Slideshare version of their presentation after a conference talk or workshop so delegates can refer back to it) and a social network (people can comment on presentations, favourite them, Facebook Like them, Tweet about them and so on).
Why use it?
Like Scribd (see Thing 17), uploading your presentations to Slideshare means simply that more people will see them, so the amount of hard work you put into creating them gets more of a reward.
Here's an example of a Slideshare presentation, embedded in this blog:
Because people like to share and engage with presentations on Slideshare, it's had lots of Facebook likes and Tweets about it, and it's been embedded on 9 other websites apart from this one - people are MUCH more inclined to take your slides and embed them on another website than they are to take blocks of text and reproduce them. This particular slidedeck was 'featured' on Slideshare's homepage, meaning it got around 2,000 views it wouldn't otherwise have had. Slideshare has enabled me to take something I presented to 30 people in a room, and get it to a worldwide audience.
Activity: set up a slideshare account
If you never give presentations, you can skip this activity - although it's still worth going to slideshare.net and searching with keywords on things you're interested in, as it's a brilliant source of information.
If you DO give presentations then sign up for an account and upload a set of slides. Tag them with relevant keywords, and if you're on Twitter tweet a link to them. You can even embed them in your 23 Things blog if you're feeling experimental! Good luck.