Xmarks, the cross-browser bookmark-syncing service is calling it quits after four years of helping web users keep their favorite sites and other relevant data neat and tidy across multiple computers. The company’s co-founder and CTO Todd Agulnick posted an open letter on Xmark's blog explaining that the lack of a profitable business model and failure to sell the company led to the decision to close down the service on January 10, 2011.

The service started its life as a Firefox extension dubbed Foxmarks, but it eventually dropped the ‘Fo’ from its name and spread to Microsoft's Internet Explorer, Apple's Safari, and Google's Chrome. Besides synchronizing bookmarks, Xmarks allows users to store their browsing history, saved passwords and opened tabs on a central server, and push them out to multiple browsers and operating systems.

The company tried various ways to monetize the information it acquired from its two million users, including a "smarter search" idea based on data gathered from its anonymized collection of bookmarks, enhancing Google's results or even selling ads to ranked sites -- but apparently none panned out. The prospect of finding a viable business model seemed even more unlikely as similar bookmark-syncing services were built into browsers like Firefox, Opera and Chrome.

For those of you who are sad to see the service go, the team has compiled a list of alternatives for all major browsers, including Firefox Sync, Chrome Sync, Windows Live Essentials for Internet Explorer and the $99-per-year MobileMe service for Safari. If you know of other alternatives feel free to mention them in the comments.