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Chrome sync (built into Chrome)
I am constantly moving from machine to machine. I used to use Foxmarks (and then Xmarks), but like Firefox itself, Xmarks became quite cumbersome. Even so, syncing my bookmarks was necessary. Also, back on Firefox, I tried to keep my add-ons in sync, and so for a while I tried FEBE, but was never very happy with it.
Chrome sync, on the other hand, rocks the house. It syncs bookmarks and extensions seamlessly, without any hassle. It just works.
The only thing I don't like is how Chrome syncs bookmarks between desktop and mobile versions. The version of Chrome on my Nexus 7 and iPad seems to have some strange desire to provide both mobile and other bookmarks, and wants to display them in a grid. Just weird. Even so, it's nice having my bookmarks on the mobile device. Maybe someday, I'll get my extensions running on my Nexus 7 (hint-hint).
How often used: Constantly.
Live without it factor: It makes Chrome my must-have browser of choice.