want to use Session Variables in Sharepoint?

SharePoint by default does not use session state. You need to explicitly change it in the Web.Config file of your sharepoint web application
<httpmodules>
<clear>
<add type="System.Web.Caching.OutputCacheModule" name="OutputCache">
<add type="System.Web.Security.WindowsAuthenticationModule" name="WindowsAuthentication">
<add type="System.Web.SessionState.SessionStateModule" name="Session">
</httpmodules>

and also

<pages enablesessionstate="true" enableviewstate="true" enableviewstatemac="true" validaterequest="false">

LINQ, .NET 3.5, Visual Studio 2008

Sick of unexpected error in sharepoint? Want to see asp.net like error page with stack trace?

In the web.config of the sharepoint web application set the following settings
"<"SafeMode MaxControls=“200“ CallStack=“true">"
"<"customErrors mode=“Off“/">"