The Use case for this Scenario would be, Avoiding duplication of Leads so how does it works? This would be helpful while Creating a Lead Search/check whether Related Account exists or not in Salesforce, if found go ahead edit else click Create Lead Tab to Create a new Lead record.
How Visualforce page looks like?
Please check source code at following links
Visualforce Page, http://bit.ly/leadsearch-vfpage
Apex Class, http://bit.ly/leadSearch-apexClass
Please feel free to comment below If you're facing any issues