Xmarks, a browser sync service with two million users is shutting down in Jan. 2011. They recommend a number of alternatives, including Firefox Sync, which I’m using now. It too will sync bookmarks between multiple browsers as well as passwords, and also saves history and tabs. Yes, tabs.
The Firefox Home app on the iPhone is the companion app Firefox Sync. You can access all your bookmarks and open tabs as well. Nice. Both are free and by Mozilla. Sync will be built-in with future versions of Firefox.