A friend of mine accidentally shot a load of photos with the white-balance on his camera set to "Tungsten". Is there an easy way to correct the resultant colour cast with Photoshop?
You can load jpegs into Adobe Camera Raw from bridge and have all the same options as processing a raw file. So you can choose the correct colour temperature and then convert several files with the same settings.
The level adjustment feature in Photoshop has the droppers for picking the white, black, and neutral points from the image. That would be quick approach, though you'd probably have to play with it a little from some images.
I do this quite often in Paint Shop Pro, using the Manual Color Correction feature. By consecutively right-clicking and left-clicking on the same patch of the picture that should be white or gray, you end up with "before" and "after" colors that are identical. Then you modify the "after" so that it has less saturation, taking it closer to pure white or gray. If you don't have anything in the picture in the white to gray range to sample from, you can sample from other colors and make them match one of the built-in samples or any other arbitrary color.
Sorry that I don't know how to do the same in Photoshop.
If you don't have Photoshop (some of us prefer to use the camera to create the art rather than the computer) then there are other options. I use Nikon's NX2 software for all post processing, it's very quick, clean and easy. You can download a trial that has no limitation other than a 60 day limit from Nikon's web site ... http://nikonimglib.com/cnx2/
To correct the white balance you need to find something on each image that is meant to be a shade of white and then use the Neutral Control Point and click on it. I have found this corrects over 90% of any white balance issues with a single click!