Social Bookmarking Sites
Each icon at the bottom of the pages on this site relates to a different social bookmarking site. Click on one (OnlyWire please!!) and you can bookmark pages on this site - provided you have an account at the relevant sites. If you haven't, you can immediately create one.
Quality of Backlinks
One of the reasons that Digg carries such weight is because of their aggressive stance on spamming. [My page on Avoid Getting Banned will provide you with some guidelines and tips to ensure that your backlinks don't suddenly disappear - as happened to many an enthusiastic social bookmarker.] There are around 30 high quality bookmarking sites where you can expect your IP address and bookmarking patterns to be monitored.
At the very least, you should have an account at each of the most important sites. (and Roboform can save you lots of time in creating these accounts) Spend some time bookmarking sites that are of interest to you, as well as the sites that you are looking to promote.
Backlinks - outside the Top 30
There are many lower-ranking bookmarking sites created by installing freely available software on a web server. The most common are know as Scuttle, ScuttlePlus, Pligg and AutoPligg
Such sites are very easy to setup and so there are thousands of them. A few have a PageRank score but many do not. I'm not convinced about the value of links on these sites. Few humans will actually use the sites - why would you when there are the vastly superior Tier 1 sites - and that means that Google is unlikely to give them any weight.
On the other hand, on the basis that there are a few sites with PR amongst them, perhaps we shouldn't ignore them completely - if it doesn't take too much time = automation.
Follow this link to find out my approach to social bookmarking.