Here are a few applications and services that will allow you to keep your bookmarks and favorites automatically synchronized between different web browsers such as Internet Explorer, Mozilla Firefox, Safari and Opera on Windows, Mac OSX or Linux, across multiple computers.

SyncIt: 2Go provides a free client for Windows called SyncIt that can automatically synchronize bookmarks and favorites from IE, Firefox, Opera, Seamonkey and Netscape across multiple computers. Apart from synching, you can also view these bookmarks from any web browser using the My2Go service or from a mobile device using 2Go Mobile.
Download SyncIt here.

Bookmark Bridge: Bookmark Bridge is an open source GUI based utility Linux & Windows and supports merging bookmarks and favorites between Internet Explorer, Opera, Mozilla Firefox, Mozilla Firebird, and Konquero. It reads bookmarks from each of your browsers, looks at the differences between the bookmarks on each browser, and saves the differences back to each browser.
Download Bookmark Bridge for Windows and Linux from here

Bookdog: Bookdog is an application Mac OS X that can sort, organize, eliminate duplicates, automatically verify, migrate and synchronize bookmarks between Safari, Camino, Firefox, OmniWeb, Opera, Shiira, and Google Bookmarks.
Download Bootdog for Mac OSX here

Bookmark Converter: Bookmark Converter is another bookmark synchronizer for Windows supporting all major web browsers including Internet Explorer, Netscape, Opera, Mozilla, Firefox/Firebird/Phoenix. It converts bookmarks in any direction between any browser types, detects duplicates in any bookmark collection and lets you search for, and browse/edit, any bookmarks in any collection. Advanced users can use the command line functionality of the software.
Download Bookmark Converter here

CoronaSync: CoronaSync is a free software for Mac OSX and Windows by Coronasoft to synchronize IE, Firefox, Opera, and Mozilla browsers favorites and bookmarks across multiple computers. Apart from that, you can also synchronize audio, video, documents, files and folders across multiple computers.

Safari Bookmark Exporter: This is a free utility for Mac OSX that allows you to export boomarks from Safari into Firefox, Camino, OmniWeb, Mozilla, IE, iCab and Opera. It allows exporting to multiple browsers in one pass, adds automatic replacing of bookmark files, and warns if web browsers are active before exporting. Currently, you cannot import boomarks from other browsers into Safari.
Download Safari Bookmark Exporter here

Sync2It: Sync2It is a free software for Windows and Mac OSX to synchronize bookmarks between IE, Seamonkey, Firefox, Netscape and Opera. The Mac OSX version only supports Internet Explorer and Safari web browsers.
Download Sync2It for Windows | Mac

Furthermore, each individual browser has addons to keep bookmarks and favorites synchronized to an online server.

+ Opera 9.5 Kestrel has introduced support for synching Bookmarks and Favorites to My Opera
+ Mozilla Firefox for Linux and Windows has an addon called from Google called Browser Sync to synchronize bookmarks and favorites with your Google account. [link]
+ For Internet Explorer on Windows, there is an addon by Ie7Pro which can be downloaded here.
+ For Safari on Mac, you can utilize iSync to synchronize your bookmarks and favorites to a .Mac account.