The Olfa Advantage rulers are the best thing since rotary cutters. They're frosted so you can easily see on both light and dark fabrics, non-slip for stability and have thin lines for precision cutting. The 6x24 ruler has angle markings for 15º, 30º, 45º, 60º and 90º...way more than most...that work for both right and left handers.
http://www.olfa.com/RulersDetail.aspx?C=66&Id=146