http://www.aqpm75.dsl.pipex.com/hullgen.html
Now this is an old link I had, I don't have shockwave right now so I can't test it out so who knows if it still works. It makes stuff that is good enough I guess. I've never been a fan of the "random squares" theory of detailing. I say make it look REAL, give it a purpose but, eh, it works. I blame Star Wars.
Best way as far as I'm concerned is to make them from scratch - It's not hard and you can do it with free software like the GIMP. If you're actually interested I will elaborate on these techniques otherwise you get the 2 minute version:
An easy way to get that hull plate look is to start with a base color and start scoring lines with the burn tool (1px reasonably hard brush) and then you can select those plates and use a large fuzzybrush and poaint littel gradients in the corners to give them that classic weathered look if you want (silly, no weather in space)
or alternatively draw lots of little square selections, paint them in (no fills, by hand) and repeat, occasionally fading layers down to almost nothing, adding filters and or cranking the contrast all the way, fades and etc and bam you have some hull plates.
The advantage to doing textures yourself is that it will always fit your ship, and you can add little hatches and signage and nose art or whatever and everyone will love you and bring you nice things and you will maybe become rich and famous.
EDIT: also do what Periculi says, once I made a planet texture from a bowl of corn flakes.