dompdf is an HTML to PDF converter. At its heart, dompdf is (mostly) CSS2.1 compliant HTML layout and rendering engine written in PHP. It is a style-driven renderer: it will download and read external stylesheets, inline style tags, and the style attributes of individual HTML elements. It also supports most presentational HTML attributes.
PDF rendering is provided using a modified version the R&OS PDF class written by Wayne Munro, http://www.ros.co.nz/pdf/. (Some performance related changes have been made to the original.) Eventually there will be support for alternative rendering backends (PDFlib and ClibPDF, for example, or even image rendering with GD). Using the R&OS PDF class, however, eliminates any dependencies on external PDF libraries.
dompdf is entered in the Zend PHP 5 Contest. While the current release is only version 0.3, it is quite usable and has been adopted by at least one enterprise PHP application.