theres nothing wrong with game modding, it helps us to realise what comemrcial quality should look like.
Half-life, you will need to look up SMD importers, Milkshape can do this, but not for HL2 (i think) and there are 3ds max importers everywhere, same applies for Doom 3/Quake, with MD5 and Anim importers.
you can find the models and there maps in the .pak archives (games such as D3 and FarCry) which can easily be opened with WinRAR
i havent tried Battlefield 2, but i should assume EA want to guard their media more than others, could be wrong though, google it.
p.s guys, he knows he cant sell them or distribute them freely.