A beautiful add-in is Sparklines for Excel. I always want to use it but I can seam motivate it as useful.
One thing I always ways annoyed about was the Pivot-tables inability to refresh from old variables. PivotPower can help with this.
Next is a Optimizer which I would have had a great use for when I studied at the university.
Today it's more of a solution to problem I don't know of yet.