Do these things really work? Have you seen any ranking inprovement or does it just send traffic to addthis which increases their ranking?
Social bookmarking sites are helpful in some ways: they can bring targeted traffic to you page; they are great way of branding; social bookmarking sites are often spidered (one my site was indexed by Google the next day I added the bookmark at Ma.gnolia.com) links from them are one-way backlinks.
If you are using wordpress you can use ShareIT wordpress plugin as well, it is pretty simple; http://alexking.org/projects/wordpress
I guess that addthis is the most used one. And if I tell you that I use it as well it won't be something new that you hear.
Its a button that provides link to all the large bookmarking sites. Javascript. It's pretty easy to see it in action on addthis.com If you are on phpbb boards - there are also nifty mods to integrate the largest. This goes again for most blogging systems, forums and CMS.
I use addthis.com although it causes my pages to fail W3C validator so I can't place the W3C logo on my pages lol