I've done a few, here goes...
A1000:
1Mb internal RAM hack
Kickstart ROM hack
PAL grounding hack
A500:
Onboard memory expansion by population existing slots on rev 6.x boards.
512k chip/fastRAM switch mod
Fatter Agnus upgrades on earlier boards.
Homemade IDE interface (2 working prototypes see images section)
14Mhz accelerator project including using square 68000 from A600
68010 upgrades
A1000 to A500 keyboard hack including mounting A500/GVP HD8+ board,HDD, CD-ROM and floppy drive to bottom of desk for zero footprint computing also using AT PSU.
A520:
Hacked VGA out
A1200:
PCMCIA reset hack
A2000:
Added full size AT slots and used AT backplane 386 (*see below) plus expansions like CD-ROM and HDD and used PC2Amiga software to access them from the Amiga side, all within the same box.
Supraturbo 28 for A2000:
Overclocked from 28MHz to 40MHz
A2088:
Overclocking hack
*286 backplane CPU card:
Upgraded with 386SX CPU
Many more PC and other C= equipment hacks/repairs/mods that I've deliberately left out or forgotten.
