It's called Eclicps. It appeared in http://slashdot.org today.
Heres the link to the story and commentary: http://games.slashdot.org/article.pl?sid=05/06/18/175235&tid=206
Personally, I don't have HL2, but with the awesome graphics this thing shows, the system (you throw stones and objects at monsters and stuff), and the fact that this was made by students, when I do get HL2, I will make sure to try it.