Hi Keith ... if you have a look at the dropper icon on the toolbar, you'll see that it has a little triangle in the bottom right (?) hand corner.
If you mouse over or click on the dropper and keep holding your mouse button down, it will bring up a drop down box. The bottom option is the measure tool. Release your mouse on the ruler and there you go!!
This would have been much easier if I was at home with my version of PS .... sorry!
I wrote this and then saw the other answers. This is in answer to your original post:
Keith, I don't think so. I've upgraded to CS2 and I thought that they had included a new tool there, but I can't find it either.
Display the ruler (Control R) and then drag down or from the side to get a horizontal or vertical line. Drop it where you want it, because I can't figure out how to move it after it's dropped.
Double click on the background layer title if you're still in that layer and change the title so you can transform. Use control T to call up free transform and then use the little corner arrows to move the picture around and get it straight. Accept the change.
LOL, you lost me there. The first time I just used the measure tool, draw the line on my image the way I wanted (what I wanted straight was at an angle). With the "guide" line on the image, you can go to Image>rotate canvas>arbitrary. It will auto fill in the angle that you need in order to make you line level. Worked like a champ for what I needed!
I walk a lonely road
The only one that I have ever known
Don't know were it goes
But its home to me and I walk alone
Measure tool ....draw a line along the horizon
go into rotate , then arbitary , you'll find the degrees already in , just click OK.
Remember if the horizons way off you'll have to crop a fair bit out .
If you can't take it back...........Don't give it out.