Social bookmarking is an important part of generating traffic for any site, it allows you to spread your content virally and having an easy way to do it is an important step, because if it's difficult, people just won't bother, here are 4 great social bookmarking plugins for WordPress.
Addthis - One of the best and most widely used social sharing plugins for WordPress, it gives you several options for the layout of your sharing buttons. In most cases the Facebook and Twitter sharing buttons will be focussed on along with the "Addthis" button which enable your blogs readers to share your content on 295+ sharing services.

You can also sign up for a free Addthis account which will then give you access to an analytics dashboard on the Addthis website, which will show you exactly how your content is being shared, for example - what services are being used to share your content, which pages are the most popular and how many clicks you are getting from when your content is shared.

Share This - Another very widely used sharing button, like the Addthis WordPress plugin, ShareThis gives you access to an analytics dashboard, on the downside it takes slightly more time to setup and the information isn't laid out as straight forward as with the Addthis Analytics dashboard. Also the ShareThis plugin will only allow sharing to 50+ services at this time, compared to the 295+ that Addthis allows, but whether this will actually have an impact is debatable, because over 50 sites is quite a lot anyway in my opinion.

Sociable - This plugin allows you to add social bookmarking buttons to your posts, pages and RSS feeds, there are around 99 different social bookmarking sites to choose from currently, on the downside there is currently no form of analytics for this plugin unlike AddThis and Share This.

AddToAny - I think this has to be pretty much the best social bookmarking plugins for WordPress, although while it's buttons don't look as fancy as ShareThis or Addthis, it has additional features that set it apart from other plugins. One of these features is the smart menu, this means that it will place the services that visitors use at the top of the menu, making it much easier for users to share on their favourite sharing service, it also gives individual share links, shares to over 100 different services, so it's a lot more than ShareThis, but not as many as Addthis, but come on the 295+ services that Addthis share to is probably a little over kill anyway.

Another great feature is that you can integrate this AddToAny plugin with your Google Analytics account, which makes viewing the analytics for your website that bit simpler as everything will be in the same place. All in all this is a great plugin, not as flashy as some, but has a lot going on under the hood.