How to get current user who’s accessing an ASP.NET application?

The quick answer is User = System.Web.HttpContext.Current.User

Ensure your web.config has the following authentication element.

<configuration>
    <system.web>
        <authentication mode="Windows" />
        <authorization>
            <deny users="?"/>
        </authorization>
    </system.web>
</configuration>

Further Reading: Recipe: Enabling Windows Authentication within an Intranet ASP.NET Web application