Yahoo! Site Explorer has officially merged with Bing Webmaster Tools. This means the day of checking your site’s backlinks or inbound links with Yahoo! are over in essence. You can opt to install tracking code on your site and use Bing, but I personally found it cumbersome.
It took me a while to understand how Bing’s version works. But now It seems that Site Explorer was much more user friendly and perhaps more accurate.
Fortunately, there are some other sites that do check backlinks such as Backlink Watch which I find highly effective. Also, using Google Webmaster Tools you can track inbound links and other data for your site.