Despite not being familiar with Sitecore, I'm going to go out on a limb and suggest that Drupal is a pretty good match, exactly because it is insanely flexible.
When looking to migrate a church website from static HTML to a CMS platform, I downloaded and trialled over 50 different platforms. Took my weekends for several months- yeah, I'm a sucker for punishment! Most I could discard in less than half an hour - stability problems, lack of documentation, no community to speak of, and so on. Drupal was my final choice.
The default configuration of Drupal is blog-like, sure, but that seems to be mostly because that's a configuration new users understand and can use as a staring point for customisation.
A key ability that I like is the way that Drupal treats all content the same - allowing you to show multiple perspectives of the same content from different places within your site.
Some modules, such as CCK and Views, are so powerful as to require independent study in their own right. As you can guess, this is both a good thing and a bad one.
I'm a full time developer - but have almost never needed to crack the hood and worry about PHP code.
If you want a prepackaged solution with full support - something close to the closed-source model - check out Aquia Drupal, they offer a full installer and support with some stunning extras. (Clarification: Aquia's Drupal distribution is still GPL, so you can download and trial/use it for free. They just provide support and some network services as well).