Mod DB offers downloads of a prerelease version of the Strider Mountain modification for Half-Life 2, offering the first five levels of this 15-level modification with a side-story from the saga of Gordon Freeman. There is an accompanying Q&A with the team describing a roll-back from the Orange Box engine due to bugs, which is unfortunate, since it sounds like this was what was addressed by the recent Orange Box engine patch. Thanks Ant.