Err.....lets see you make Half Life 1 using the source engine....
Not to make this a discussion of this slightly OT matter but are you not forgetting that Half Life is made by professionals?
What I meant to say is that I can hardly compare the scope of mods released for MW on an every day basis to anything on the internet as I know it. I love modding, Quake has many but it's not as active, Unreal has many and they are a really talented bunch but yet again, not as active at least in the rate of released mods. I have no real data to back this up either then experiance which kind of does not support my idea.
What I mean when I say scope is only the scope of release mods per day basis, and also a bit comparative to the year of the release of the game.
Also, Source engine has many mods, and Valve makes good use of the modders, making their work seen through steam, a real win win but I'd say MW would still have an edge.
Again, many modders in other communeties are very devoted and work hard, but nowhere else have I personally witnessed such scope, everything from some releases per day to 3rd party programs, new discoveries from time to time etc.
Not trying to argue here, everyone can have their opinion of course.