For 2012, XMarks came back from the grave as part of LastPass. It supports 4 of 5 major browsers (IE, Firefox, Chrome, Safari) but lacks Opera. It's not clear whether this is due to Opera API issues, but there is a little discussion in their forums. Looks like it's free, but you have to subscribe for use with mobile devices/tablets (iDevice, Android, etc.) and a few other features. This is the same strategy used by LastPass. (Disclaimer: premium user of both)
Google Bookmarks may be an option depending on your browser choice and plugin availability. Google Chrome Sync (and possibly plugins for other browsers) may also work; for Firefox I believe it's tied in with the Google Toolbar for Firefox but YMMV by browser.
Delicious may be an option; I used to be a user but have mostly stopped after the headaches with their switchover (I switched to XMarks). I think they were trying to go in a more "social" direction like Digg, et al.
Pinboard is a commercial service (one-time fee) that apparently also uses the Delicious API, so some addons/widgets may be able to work with it instead of with Delicious. Long-term support of those may be questionable if Delicious doesn't still support that API.
UPDATE: For XMarks, I'm not sure what their plans are for IE10, but I'd expect them to only support the Desktop version of IE - there's apparently also a "Metro" version that will not allow any plugins.