A useful addon that gives you casting warnings for nearby enemies. Nice to have in arenas as you get to see what your non-targeted/non-focused opponents are doing. By default the warning message takes the format of ” [spell icon] ENEMY –> ENEMY TARGET” but this can be changed in the options to display the full name of the spell being cast (good if you never realised that the icon for Chaos Bolt looks like a turtle being sick).

More info and download link on the curse site.