For me, twitter, digg, stumbleupon, delicious and mixx are top 5 services.

Have a nice day,

Delicios, Propeller and reddit too

digg, mixx, reddit, squidoo, slashdot=dofollow

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Hi,
According to me digg, mixx, reddit, squidoo, delicious and stumble upon are the best social bookmarking sites.
One of the members mentioned twitter here in social bookmarking sites but twitter is an social networking site, It;s not a social bookmarking site.

digg, stumble upon, delicious

My option ares Stumble upon and digg both gives me hundred of visitors daily.

It would depend if you are utilize it for SEO efforts or not.
Some of the high PR do-follow social bookmarking sites are Digg, Faves, and Spurl.

use the following, you will get good results,
facbook.com
digg.com
mixx.com
buz.yahoo.com
twitter.com
delicious.com
folked.com
propeller.com
stumbleupon.com
and faves.com