There are 423 questions tagged effect. This seems to be a completely non-tag, that should be removed. It looks like it's being used for "How can I get this wavy effect?" type questions.
I think this suggested edit to the tag wiki sums up my argument best:
Effect is a result or change of something. For example, a graphical effect (adding sepia to pictures, page curling), sound effects (high treble/bass, changing pitch), etc.
i.e. almost everything.