Foxmarks

Xmarks for Internet Explorer
FreeFind, synchronize, and back up your bookmarked sites.
WindowsXmarks for Firefox
FreeKeep Firefox bookmarks synchronized and discover new sites.
WindowsXmarks for Safari
FreeAdd-on that allows you to manage and sync your bookmarks across browsers.
MacXmarks
PaidBring your desktop computer's bookmarks with you wherever you go with this handy syncing utility for iPhone.
iOS