Yeah, I know its old, but I just started and finished playing Minerva, a Singleplayer mod for Half-Life 2 Episode 1.
I must say, the mod is of very high quality, I was overwhelmed by a particular section of which I won't mention just incase there are folks who are behind like me. But its no surprise that Adam Foster, the person behind this mod now works at Valve because they took such interest in this mod.
Here is the ModDB link for those interested. Just make sure you know the Half-Life story to a farliy good degree otherwise the "overwhelming" part won't be so awesome.
I find it quite interesting, that Foster took the approach of designing the environments first, than adding the gameplay, because I thought the gameplay was really well done. I must admit, that seems to be my approach (I think I just get carried away), but it doesn't seem to work for me as well.
Anyway, outstanding environments and gameplay, great story and good use of ambient sound. It gets a 10/10 for me, probably the best mod I have playin.
Aaron's Stuff