Been searching for some too.. doesn't seem to be many (any I'd use anyway) all seem to be some dodgy looking American websites made by a 5 year old.
Social bookmarking is a fantastic way to generate traffic to your websites and increase your Google PageRank, I've just started reading up on it this week.. and it looks quite promising.
Please do let me know if you find any software, that works.
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Bristol Web Design - Christopher Darling
www.christopherdarling.co.uk