This must be extremely easy to implement into the builder in SecondLife!
When you make a prim in SL and want another one like it, only "mirrored", you have to get in and change the rotation values of it in the editor.
Although relatively simple, once you get a grip on how it works, it still takes up unecessary time.
My suggestion is to add two check-boxes to the object tab - One saying mirror horizontal and the other saying mirror vertically, similar to the two in the Textures tab (flip horizontal/vertically).
When checking the box, SL automatically changes the rotation value of the object to it's opposite. This can't be too hard to implement - It is after all just the same function as a calculator.
If this was implemented, it would make building easier for beginners, faster for advanced builders and would definitely increase the overall quality of builds in SL - After all, even the best of human minds can miscalculate a bit!
What do you guys think?