I came across a great article today written by Miguel Sousa about the benefits of OpenType (.otf) over TrueType (.ttf) fonts. I never put much thought into the difference between the two font extensions, although I have always leaned towards using OpenType fonts when available knowing that is a more recent format. Miguel goes into great detail on the differences. The main difference that struck me the most as a RIA developer was that OpenType fonts can be up to 50% smaller than TrueType fonts. When embedding fonts or loading external fonts into a multilingual Flash application this piece of knowledge could trim off a significant amount of load time. Read more about the differences here.