Document Preview allows users to find information dramatically faster. User can easily evaluate a document without downloading and opening it. Previews are rendered forestall on the server side. So at the moment user hovers over the search item first page is already rendered and gets displayed immediately.
Major office formats (Microsoft Word, Microsoft Excel, Microsoft PowerPoint, PDF, RTF) and images are supported. HTML rendering support is coming soon.
Although Rebel Search Preview for SharePoint Search 2010 is a free product, it is a fully supported product.