I mentioned this last night to someone.
If you have an ATI card you can get HL2DM for free, which i believe will give you the SDK which will allow you to play Fortress Forever if you're not already playing.
I believe if you have the orange box, you don't get HL2DM so this would allow you to get it.
http://www.steampowered.com/ati_offer1a/
You will need to access it with either IE or Firefox IE Tab