Plaxo.
Funny, I had actually forgotten about Plaxo until this..
Got invites to join this a couple of years back (mainly to mailing lists I am on, but a few to my personal email account as well)..
I didn't sign up, because just attaching my phone to each computer and synching the contacts does a good job of keeping my contacts lists up to date on every computer I use. I don't use my contacts list on other computers, if I need to look up a contact, I look it up on my phone.
The other reason is I am naturally cautious when giving out my details. Even where I fill out details on a site I trust (like Facebook), I only put the vaguest of details on the site (nothing that is not publicly available). Even a forum I have managed for years (and know the Owner personally) only has vague details in my profile.
I would certainly not upload my contacts list to something like Plaxo..