I personally just use Yahoo's site explorer, you can choose "Inlinks" to view all backlinks pointing to a particular site/URL.
Traffic Travis is good if you're trying to build an adsense site. I personally liked link assistant, but it's not free.
If you have mozilla firefox in your system, there is a free add-on called SEO-Quake, download it. It will have a toolbar with all the details you want of that particular site. Once the toolbar is downloaded, you just have to open the site url in the address bar and you will get all the inlinks as well as backlinks it has from other sites.
I recommend the SEOpen Firefox Plugin. There you can do a mass check. It opens several backlink searches (Yahoo etc.) in separate tabs.
Use google webmasters tools. at first, yahoo explorer indexed more backlinks than google webmasters tools. but in the ends, google webmasters indexed more every link i made..
Here are a bunch of free tools you can use to check backlinks: http://www.webmaster-elements.com/webmaster-tools/link-checking-tools.htm