Most of the big five above have the same basic functions with only a little variety. There are a lot more, however, that have interesting focuses or some bit of flair that might make them attractive.
Technorati: If you like to keep up with the whole blogosphere, technorati is the authority on social bookmarking for blogs. It keeps track of what is going on currently in the blogs and lets you organize, find, and keep your favorite blogs and entries.
Newsvine: If you want to focus on keeping up and organizing the news then newsvine may be worth checking out. Even if you do have another social bookmarking site, newsvine is an incredibly helpful way of organizing and finding news. For those who want to actually become a part of news contribution, you can even opt to write your own column and articles for this site (and get paid for views).
Kaboodle: If your one of those people who loves to keep an eye on all of the latest products and deals online, Kaboodle offers an excellent way of socially finding and sharing products. You can bookmark products, review them, rate them, comment on them, then share your list with friends.
Propeller: If you are an AOL user then it may be good to know that AOL has their own bookmarking site that, due to the number of people still using AOL, does well enough. However, its features offer nothing that you couldn't get from the big five. Only convenience is the integration into your AOL account.
Mixx: If you feel like you need more direct say in what content is delivered to you then Mixx provides you a bit more freedom to cater your newsfeed as you like it. Unlike the recommendation algorithms that Digg and StumbleUpon use, this gives you more direct control over the links it brings up for you.
Fark and Slashdot are both culturally-oriented, editor-filtered social bookmarking sites. Fark is oriented towards a young, male, and somewhat crude audience. Slashdot is oriented towards nerds and geeks. Both of these have a series of editors that control, to some degree or another, the content that goes up. They both have been around for a while and have a large community, however you have to pay to get the full usage of Fark and Slashdot normally assumes a bit of technical knowledge.
Diigo: If you are the kind who obsessively annotate everything you read, Diigo (a somewhat obvious rip from Digg) has one really unique and cool feature: it lets you post notes and annotations over sites, save them for later, and share them with your friends.
Twine: If you want to find people with your same interests, with whom you can share links related to that interest, try using twine. It is organized around different "twines" that are like clubs for a particular interest. When you put in a link you submit it to a specific twine. This neatly parses out links and lets you be part of a group built on mutual interests.