A lot of people use the big sites like digg and stumble. I use those sites as well, but I like to focus on some of the smaller to medium sized social bookmarking sites.
I can submit a link to digg, and it may never show up in my google webmaster tools. On the other hand, I can submit a link to mixx, and its indexed within a few days or even weeks.
Also, I like the commenting system that mixx has. When you post a comment on a submitted link, your name is linked back to your profile. Its like leaving a bread crumb trail back to your submitted links.