Yahoo recently announced they will be closing the doors on Site Explorer one of their most popular services among webmasters and SEO fanatics. Yahoo Site Explorer has been around for nearly 6 years and many people have come to rely on the service to check inlinks/backlinks.
Yahoo has been slowly eliminating features and services, but this one was a little unexpected. Yahoo suggests using Microsoft/Bing Webmaster Tools after they shut down Site Explorer. Which likely has to do with the deal Microsoft and Yahoo have with search results powered by Bing.
Bing has been updating their webmaster tools which include a few new features. You can now find a similar feature that provides Inbound Link details in your Bing Webmaster account, which is a plus. But, since you have to login to see the report details I don’t know that you can check sites that you do not own or maintain.