Probably not on the "simple" side, but for more sophisticated ad-hoc reporting that allows business users to build a report in the browser without knowing SQL and without installing anything, check out our ActiveReports Server product.
We built it entirely in managed .NET code and we include ASP.NET controls to make it easy to integrate into your own ASP.NET sites. Of course there are webservices for other languages/platforms, and it takes care of many other important details for web-based reporting like scalability/load balancing, scheduling reports, output to excel/pdf/word, etc., role-based security, multi-tenant deployment support, mobile device support, and it is compatible with ActiveReports Developer Component XML report files.
And yes, I'm the Product Manager.