I just found out about these guys. I’ve used the Scale Rule in the past and I do like it, but, it is black on clear and I sometimes have difficulty reading it. They offer rulers in both acrylic and aluminum. I ordered the aluminum, in both 6" and 18". They also offer a 24". They are thin aluminum (.020") so they do have some flex in them, which I find quite handy. To protect them in shipment, they also included a steel bar - I’m sure I’ll find a use for that as well. 
One cool thing is that you can specify the scale that you want on each side. I went with 1:20 on one side and 1:1 on the other. The choices include HO, S, O, 1:1, 1:12, 1:20.3 1:22.5, 1:24, 1:29, and 1:32. Here’s a picture of the 6" ruler:
I find it easy to read and use. Pretty neat stuff. You can find them over at http://therailscale.com/ (No financial interest, blah, blah, blah. )