Ohh boy... This is my kinda thread!
For starters, here's my CPU heatsink since the fan on it broke:
Just temporary until I can get a new cooler.
My case, two 120mm fans added in the side.
There are two mods in this pic, but they're hard to see;
The 80mm fan hole on the back was originally 60mm, I've enlarged it.
The PSU fan is controlled by a 6-fan fan controller in the front, as opposed to it's broken automatic steering (Always maxed, WHOPIIIIIEEE)
The blue LEDs in the front are taken fronm an NZXT case that someone threw out, they blink and fade, really nice light show. This pic is kinda old, so the upper fan isn't there.
My laptop-picture frames, had to re-route the cables on the back of the screen to enable screen flipping.
My 7600GS; It used to be passive, now it's cooled by a 70mm fan. Never over 40C on 600/500MHz with the fan on lowest speed.
And here is my leetest ghetto mod so far: An Intel socket 423 fan on a Radeon 9600SE passive cooler my GeForce 6600. It owns. Quiet, and very effective.
I have a ton more mods, but I can't be arsed to post them right now.