Yahoo site explorer was one of the easiest way to check for back links and one of the few place where linkdomain: search was working properly. Unfortunately it was closed.

Alternatives:

• Google Webmaster Tools – but only if it is your site, on Yahoo it was working for competition, etc.
• Ahrefs Site Explorer – You can get some valuable information with a free account, but to get everything you have to pay.
• Open Site Explorer – SEOMOZ engine, you can compare up to fives sites in terms of: page authority, links, mozrank, etc.
• Alexa Siteinfo – You can see audience, traffic, but the back links information is limited.
• Dig Backlink – What I like about this site, is the way it sorts the backlinks, the most important first.