QL

Here is a visual compilation of the Old Machinery blog posts about the Sinclair QL computer and some pointers to replacements.

Display: RGB TTL, RGB Scart, Composite Video, TV. Connecting to 1084S.

RWAP Keyboard membrane replacement
Extension slot: Tetroid Disk Interface or Gold Card (QubIDE interface, Memory/CPU expansions, Super Gold Card)
HxC Floppy Emulator, connected to the TDI / Gold Card. Image format.
QL-SD Microdrive replacement
Minerva ROM upgrade, alongside the QL-SD
EPROM Cartridge -  e.g. Toolkit II ROM
Serial cable and PC file transfer
Network cable for linking two or more QL computers

Here's a table of some of the more notable modern peripherals and their features :

                      DI       GC       SGC       QL-SD
Card Reader:          YES(CF)  NO       NO        YES(SD)
68000/Acceleration:   NO       YES      020       NO
Memory:               800K     2MB      4MB       NO *)
Disk interface:       YES      YES      YES       NO
Battery-backed clock: NO       YES      YES       NO
Toolkit 2:            YES      YES      YES       NO
Minerva ROM:          NO       NO       NO        YES

*) The file allocation table requires memory

DI   = Tetroid Disk Interface
GC    = Tetroid Gold Card Clone
SGC   = Tetroid Super Gold Card Clone
QL-SD = SD Card reader