I will try to show you through this tutorial how to create a metallic looking texture. There are probably another hundred ways to do it so you must play around with layer options, blending options and filters, as well as your own imagination. The best way to create a texture is to have a real one nearby, found anywhere on the net. A good place to search for images is on Google's Image Search.
 
Now everything I did was done with Photoshop 7, and a few brushes that you can download for free on resource sites or even make your own. This tutorial will explain how to create your own. Here is how your final product will look like:

 [ your final product ]

Creating the Texture:
The following steps will explain how to create a metallic texture in PhotoShop:

1. I used a picture probably found on the net as a reference texture - as you will see this has nothing to do with the final, photoshop-made texture.
    
   I used the eyedropper tool to take the lightest color as a foreground color, and the darkest one as a background color, to use these colors as my basic gradient. You can choose anything you like for your colors.

[ the reference texture ]
 

2. Open up photoshop; create a document, RGB, whatever size you want. I kept mine small at 700x500 such because I later used a brush that had this same size.
    

3. Fill your background layer (or a new layer) with a nice, large linear gradient, I used the two tones of gray but any grey or blue tone will do - just keep in mind the colors should not be extremely bright or dark.

[ the gradient ]

4. Give your gradient some noise - I chose 6% level of noise, Gaussian (because it sounded cool) and monochromatic, it wouldn't look too nice with colors. This should give you the 'brushed' aspect of the metal.
    
   The more noise, the rougher the metal will be, if you only use a slight noise the texture will look smooth and chrome-like:

[ apply some noise to your gradient ]

5. The actual 'brushing' is done with the motion blur, very handy filter. Once again you can change anything in your filter options, but the longer the streaks the smoother the metal and the angle doesn't really matter. I chose a 12° angle on a distance of 12 pixels.
    
   At this stage you might want to crop your image because the motion blur doesn't affect the area near the borders, so crop your image leaving out the borders. It didn't show up too much on mine so I didn't crop - but I usually do with the motion blur.

[ motion blur ]

6. Now you can work on the contrasts and give the metal texture more highlights and dark tones - you can use any of the handy tools photoshop provides, I went in for a bit of contrast and played around with the curves, giving the image more light and dark colors.

[ adjust the contrast ]

7. That's it! You've got the basis of a metal texture. Now if you are doing 3d I'd recommend a few things - keeps a black and white layer following the steps, or make a new layer and give it a grayscale - this is to be used for bumpmapping - highlight maps or reflection maps.
    
   Try to make the gradient straight, not slanted, to give the texture a seamless pattern. Here is a good tutorial for making seamless textures with non seamless ones.
    

8. What should be done now is add details - give the texture more realism; that is - a proper lighting, some scratches and non uniform colors...plus a little extra at the end.
    
   For the scratches - there's only one way to do it - brushes. You can make them yourself or download them. Search on Google for rust, metal or scratch PS7 brushes. Here's a good brush site. The other solution is make your own brushes - here's a tutorial on how to do them - basically you take scratch from photos, existing pictures, etc. and give it a very high contrast.
    

9. The brush is used was made from the metal texture of the beginning of the tutorial; I used the curves to make it look fuzzy and heavily contrasted and then turned it into a brush Edit>Create Brush (eventually save your set of new brushes, can be handy for later projects)
    
   The blending options you mostly use with the brushes are the cherished dodge and burn options. What I did was use the eyedropper to pick the color of the light area, use my brush with the 'color dodge' mode and a 35% opacity.
   Since the color dodge doesn't affect dark areas I use the same brush (but changing brush would be wiser). Picked a dark color and used my brush on the bottom area with a 'color burn' and 35% opacity:

[ some further modifications to our image ]
 

10. We've got one fine texture now - and what would make it even better is a nice lighting. Now DON'T do this if your texture is to be used for a 3D model - it won't look good I assure you.
     
    The lighting effects are found in Filters>Render>Lighting Effects. Once again you must play with the position, strength of lighting (check the material percent for example). I used a spotlight with the numbers that are shown in the following image:

[ the spotlight settings ]

11. We're almost done now. The *extra* bits that I was talking about are here. I viewed sci-fi movies and noticed that ships always had a metallic grate for the hull and almost always, there was grease/oil leaking or smearing from the joints. So what you can do is re-create those 'leaks'.
     
    Here's how you do it: create a new layer, and make the rectangles in black, using lines 1, 2 or 3 pixel large. Now make a copy of that layer, and smear the copy using the smudge tool and a round brush set to 80% opacity. You've got to give it your own feel. You can even make rivets or anything else.
    Now for the final touch, take the first layer and give it a bevel and emboss - to give some depth:

[ two images: first one details the blending options; second shows the 'leaks' ]