You need this commercial program called cedega.
You need to purchase it ($5 a month for continued support/updates) at http://www.transgaming.com
After you download Cedega, you install it. they have it in source format (compile yourself), RPM (auto install package), or in .DEB format (for debian users).
After you have downloaded and installed Cedega, you pop in your Half-Life 2 CD's (or DVD) and type something like:
It will then run as if you ran the setup file in Windows. HL2 IS playable under Cedega, especially with nVidia Cards. ATI cards are not so great with Cedega, unfortunately.