I use pocket for storing everything. It's cross-browser and device so you can easily sync offline to your tablet/phone to read on a plane. http://getpocket.com/

Personally I store bookmarks as a flat file, with some javascript magic for tagging, and store that under git:

https://github.com/skx/bookmarks.public

It's portable amongst browsers locally, but obviously it's not an extension and requires more effort to work with.

(I tend to bookmark things in my browser for a few days then mass-update the git version and delete from the browser.)

I've used xmarks for several years now and I like it. Sometimes it can be awkward when different browsers' conventions both sync in (like Firefox's "Unsorted Bookmarks" appears as a folder in any other browser), but otherwise it's been nice.

Hushmarks.com is a very simple solution in that you can just add their bookmarklet to every browser you use.

Google Spreadsheet to store bookmarks, using a bookmarklet.

pinboard.in

I've heard good things about xmarks.