Lost Coast indeed requires Half Life 2 to be installed as stated.
I downloaded HL2/activated it via Steam and so could download the Lost Coast level, which is just a tech display of HDR (High Dynamic Range) Rendering for suitable cards, namely Nvidia 6800 and above and ATI X800 and above I think.
Looks pretty, but locked up my PC once and using Anti Aliasing makes the Combine Soldiers float above ground like dummys
I have the required specs, 2.2Ghz CPU, 1.0GB RAM and ATI X850XT vid card!
Not bad for a freebie but still a few bugs in it!