labelPageHeading.Text = ConfigurationManager.AppSettings("AppName")

labelPageHeading.Text = ConfigurationManager.AppSettings["AppName"];

<configuration>
    <appSettings>
        <add key="ApplicationName" value="MyApplication" />
    </appSettings>
</configuration>

<configuration>
    <system.Web>
        <httpRuntime enable="False" />
    </system.Web>
</configuration> 

<configuration>
    <system.net>
        <mailSettings>
            <smtp>
                <network 
                    host="smtp.myhost.com" />
            </smtp>
        </mailSettings>
    </system.net>
</configuration>

<configuration>
    <system.Web>
        <customErrors defaultRedirect="~/myErrorPage.aspx" />
        <trace enabled="True" pageOutput="True" localOnly="True" 
                traceMode="SortByCategory"
            requestLimit="10" mostRecent="True" />
        <compilation debug="True" />
    </system.Web>
</configuration>