If you've actually unlocked the game and had it authenticated but cannot play because steam is down do this:
1) Download GCFScape and install
2) Using this go to your steamapps folder and find these files...
half-life 2 content.gcf
source engine.gcf
source materials.gcf
source models.gcf
source sounds.gcf
Extract these files exactly as they are to your own directory - for arguments sake just make a HL2 directory on the desktop for example...
This takes a while to unpack, but it's basically all the HL2 content (you need to have unlocked to first through steam for it to work).
3) Now you need some DLL files from the CS:Source for it to run. Copy the CS:Source BIN folder to the HL2 folder that you've created.
4) Then create a shortcut to the hl2.exe file like this...
"C:\*Enter Directory Here*\hl2.exe" -steam
and (in theory) this should just run HL2 as though it was an independent game WITHOUT USING Steam - of course you won't get any updates but for now if you want to play it's a good option
