Lumicon OAG's would have to be one of the best options available today for SCT's and Newts. The fact that they integrate a focal reducing lens into the design makes them very flexible to the astrophotographer.
Though, there are plenty of options out there - you just need to do some research and work out your budget.
http://www.taurus-tech.com/tracker.htm
http://www.sbig.com/sbwhtmls/moag.htm
My only tip in using an OAG is make sure you get the right extension tubes etc so that when you reach focus with the main imaging camera, you are also able to reach focus with the guiding camera. In some cases, depending on your imaging train, the critical focus zone can be different between the two hence you'll need the right adapters to remedy this.