We have used the triggers Ted mentioned in his post as well as a wrapper for the left click bindkey as suggested by Lawrence. For what its worth, here is what we do:
The wrapper code around the left mouse button will print information to the ciw about the selected item. If it is a rectangle, it will print the layer, width, length and area. For paths it prints the width, length and layer. For instances it prints the library name, cell name, instance name and xy location. For pcells, various parameter values are printed. This allows a user to get the most commonly used information without having to query the object.
We registered a skill function with the leRegUserObjectSelectionFilter to determine whether or not an instance should be selectable. We created a user interface to allow designers to lock down cell placements. Once a cell placement is locked, you cannot select that cell (based on the trigger code we wrote). This allows us to significantly reduce those inadvertant cell movements that can wreak havoc on a design before they are discovered. We used to run into problems when routing cells might shift by a small amount and we found out hours later that many connections were mis-aligned. This trigger has saved us many hours of rework.