MiniDisc hacking
Most MiniDisc aficionados are aware of unit hacking to gain access to new features. The unit that perhaps benefits the most from this is the Sony MZ-N510, which also comes in the N520 and NF610 variants. The 2001 model R700 can be hacked to add many features of its upscale brother, the R900, as well as the Type-R codec, which renders the R700 capable of performing real-time SP recordings with Sony's last evolution of ATRAC1.
I bet the market for hacking the best music format of all time is small these days, but this is still incredibly cool.