There's only one true metal texture tutorial. Well, there's two.
First one http://www.strangefate.com/Tutorials/plate...te_tutorial.htm is probably the better one, but it's written for older games like Quake 3 or Unreal Tournament, so it includes things like lighting drawn onto the texture, which you'll want to avoid if you're texturing for something like HL2, or at least approach differently (luminosity maps and stuff).

Then there's rorshach's video tutorials, which're good but don't really offer that much explanation. Those are http://www.planetquake.com/polycount/cotta.../help_index.htm on that page, near the bottom. If you cba to sit in fileplanet ques for those, there's also a few misc texture tutorials lower down on that page, you'll wanna look at the texture tiling one, and the "rusty metal panel" texture.

The most important tip from both of those is to use photos as bases. It's nigh-on impossible to get decent looking textures without a photo or photomanipulation being involved. I suggest making several tiling metal textures using photos, then layering them above eachother, like the Strangefate tutorial shows.