Read/WriteWeb has posted an excellent review and comparison of the major social bookmarking services. Digg was not included as they are considered a social news service. The bookmark services reviewed include: BlinkList, Blogmarks, del.icio.us, diigo, Furl, Ma.gnolia, MyWeb, Shadows, Simpy and StumbleUpon.
The result? Del.icio.us and StumbleUpon were the clear winners. I seem to remember something a while back about how StumbleUpon Rocks.
Personally, I like tend to use del.cio.us the most for work and StumbleUpon for more recreational surfing. But I have found some excellent resources for work that way.
This information is pretty handy for when you creating your social bookmark links.