Here are my Top 5 (I leave all the track related stuff out), this is just for daily driving:
* aspheric mirrors (I am no longer are a threat to other drivers when changing lanes)
* upgraded radio with a Pioneer AVIC-F (Navi / iPod / Sat radio)
* Sporttech's exhaust mod + desnorkel
* short shifter
* front clear bra
Now let's talk about the most useless ones:
* add rear speakers + 6 channel amp to the existing CDR-220 (however, works great with the AVIC)
* adding "Porsche" logo to the rear windscreen (as if anyone would think it's something other than a Porsche)
* replacing the rear coat hangers with silver painted ones from eBay (I am still embarrassed about that one)