Here Mudassar Ahmed Khan has explained how to hide (remove) .ASPX extension in URL in ASP.Net.
There are two ways to do it namely URL Rewriting and URL Routing.
URL Rewrite works in IIS Servers while URL Routing can be implemented within ASP.Net code inside Global.asax file. URL Routing is supported in .Net 3.5 SP1 or higher frameworks.
In this article he has explained how to manipulate URLs by hiding (removing) .ASPX extension in URL using URL Routing in ASP.Net.