Half-Life 2 Substance features an adaptive class system and open-ended gameplay.
Back in the golden days of hl2 modding, this was something I played multiple times, and looking back at it, I sincerely wish I could go back for more :(
The new additions to the hl2 campaign struck me with surprise and awe and the dozens of new weapons made the action tight and refreshing, with new additional monsters and player abilities to decimate them with.
It's a shame this only partially works with mod fixes following the may 2010 update. Perhaps someone with some free time on their hands is willing to give it a fix. But I know that probably won't ever happen.
It's a sin and shame, but at least I still have great memories booting this up for the first time :)