It might be a good idea to compile a list/FAQ of available - and/or soon to be available - mods for the 7800 in order to better educate new retro enthusiasts as well as returning old school 7800 owners.
Off the top of my head - and not counting controller mods or Flash Memory carts - there are:
*Custom case paint jobs [Bob's various painted 7800s in particular]
*Composite Only video mods
*Composite & S-Video mods
*"Stereo" mods [or dual mono output mods]
*Power Supply mods
*"European" BIOS mod from Best Electronics
*DevOS mod
*Multi-colored Power LED mod
*Replacement [if not better] cartridge slot from Syzygy
*Power/Reset Switch mod replacements [from Best Electronics]
*XBOARD
*XM [I'm counting this due to the SIO option even if it's "external"]
Did I forget any?
*I'm not sure if anyone has done any controller LED mods like there are in NES-land.
*I haven't seen any cartridge dust door mods [similar to, say the 600XL/800XL cartridge doors].
*There's a user in one of the threads that added NES controller ports to his 7800.
*There's a user in the 5200 threads that stated he installed a 7800 mobo inside his 5200 case.