Yahoo MyWeb?

This is an interesting question. I've added such tagging links on a few sites and I can't say I've seen them used much, but I guess it depends a lot on the kind of visitors a site gets. Which social networks are most likely to work is probably also heavily dependent on your site visitors.

One thought might be to post a poll - list a bunch of social tagging systems and ask your visitors to check off which ones they use. Not only would it narrow your list, it might draw attention to the tagging links already on your site.

Seems like users would bring along the bookmarklets for the services they use.

I added delicious links to mine, but I didn't add tracking to see how much they get used. I noted a huge uptick in referrals from stumbleupon this month. (What's huge? 2500% increase.) I don't really know what's behind that, but I'm all in favor of it. I don't think you can even add a button for the stumblers, you have the have the extension installed.

Hmmm, one of my sites had a blast of stumbleupon traffic a few weeks ago too. I guess including easy tagging links serves as a reminder to the reader, even if he has bookmarklets set up. The results seem hard to predict, though.