Nice comparative article here on Read/WriteWeb about social bookmarking sites.
As might be expected del.icio.us is streets ahead in the race and has recently undergone a bit of a make over.
Casual visitors to the site might not notice the new features as they are not obvious if all you do is visit your own bookmarks page.
An interesting feature is the networking page, bringing together the bookmarks of one's friends and colleagues. This page also shows reverse networks or those people known as fans who have listed your bookmarks (I think).
I was startled to discover that I have two!
It's a bit like being stalked.