Oracle Discoverer like app native for SQL Server?
SQL Server 2005+ comes with an ad-hoc reporting tool called Report Builder, which does much the same sort of thing as Discoverer.  This will probably get you what you need.

SQL Server It also comes with a reporting tool called Reporting Services (SSRS), a SQL based tool that does the same sort of thing as Crystal Reports or Oracle Reports.  SSRS will consume a variety of data sources, including Oracle.  One key point to note about SSRS is that you can use it as a medium for publishing reports built with Report Builder.