Getting a single value from the DB
public static String GetConnectionString()


You shouldn't have to use this outside of the class so make it private. Furthermore I would turn it into a property and set it from the static constructor -- that way you avoid having to query your configuration each time you want to make a new call.

This is assuming you don't do it deliberately like this so you can change the connectionstring without halting the application.



cmdTxt


What's that, commandtext? Something more descriptive would be helpful, perhaps just "command"  or "query" would be okay.



return command.ExecuteScalar() ?? null;


"if execution returns null, return null".

Might as well just return command.ExecuteScalar();.



string.Format("SELECT User: {0} Base: {3} SQL:{2} Error:{1}"


Keep the indices in ascending order -- it's what everybody expects and you won't have silly mistakes because the arguments passed in expected 0-1-2-3 instead.



Why use a try-catch and not a simple if(Session["ClientBase"] == null)? Exception handling is expensive and shouldn't be used for normal application flow.



GetValueAsBoolean
GetValueAsString
GetValueAsInt
GetValueAsDobule


I would consider making this a generic GetValue&lt;T&gt; and do the conversion inside that method while throwing a NotSupportedException for types that you don't want to support. The above idiom smells too much like Java and we're better than that.

Notice also the typo Dobule.