Here is a list of 24 "dofollow" bookmarking sites. (Note: some of these may have changed to "no follow" by now.)
That means if you open an account and bookmark sites using these bookmarking services, you will get some link value from your bookmarks. This can be very significant if you are trying to get some SEO value out of your bookmarking efforts.
You can also increase the link value of your public bookmark archives by providing some links to them. Here, for example, are two of mine:

Stumble it!
I haven't been into social bookmarking lately.
Aside from the SEO benefits of SB sites, they can also bring in direct traffic from time to time . So its a very good investment IMO.