For my '66 Mustang? When I had it painted, they shaved all of the emblems and the antenna. I installed a Clifford Performance header (with true dual exhaust outlets) on the 200ci 6cylinder engine, and had the local exhaust shop install a couple of glasspacks. It sounds very nice. I also installed a Pioneer CD player and speakers, so I had something to listen to besides the exhaust. Other modifications include: Pertronix Flamethrower coil and Ignitor electronic ignition, power steering setup, 14x6" chrome reverse rims with "moon" hubcaps, '73 Mustang fuel cap, 14" steering wheel from Grant, Delta H4 Xenon headlamps, a nice little flaming pony mounted flush to the '65 grille, and recently added, a 95 amp high output alternator.
My '01 Mustang GT? purchased recently. It's still stock (and under warranty) so there's nothing great about it, except for the fact that it's mine. ;] I think the two cars go well together. I still drive the '66 more often than the '01 GT.

Flaming Horse.. might be able to see the Xenon headlamps also.

Here's a shot of my horrible interior... :l Only thing's I added are new front seat covers, the middle humphugger armrest thingy, foot gas pedal (I love that), and a couple of things I mentioned above. Other than these things, it's completely original.