XProc Engine version 1.0.9 has been released today - download from here. The new version reflects the latest changes to the XProc specification (see the not-yet-published Editor's draft) and improves the overall level of compliance. As of today, XProc Engine passes 99.81% of the XProc test suite (one failing test).
Except for fixing various bugs found in the previous versions, version 1.0.9 introduces a number of new features. The most significant one is probably the inclusion of the Saxon plug-in that adds XSLT 2.0 support to the engine; a feature that was much needed. Support for XSLT 2.0 also represents an important step towards complete support of the XProc specification.
Speaking of plug-ins, the plug-in architecture has been completely redesigned, with the aim to make creating extension plug-ins easier for application developers. The new plug-in infrastructure also makes it possible to use the plug-ins from the command-line, which the previous versions of the engine didn't support.
Other significant new features include the new security API (which allows application developers to contol various security aspects related to running XProc pipelines, such as accessing resources, importing XProc pipelines, executing steps etc.) and much improved, from-the-ground-up reimplemented, support for XML serialization.