Classic User Port Peripherals:
BodyLink EMG Device
Scanntronik Handyscanner 64
The Print Technik Video Digitizer
PAL Colour Digitizer Frame Grabber
Microtext Teletext adaptor
RTTY-CW Interface for Radio Communication
Promenade C1 EPROM Programmer
DELA Eprommer EPROM Programmer
Datel Eprommer 64 
Micro Maxi Prommer EPROM burner
Auerswald ACC-64, a DCF-77 time signal receiver
Commodore 1600 modem
Commodore 1650 modem
Commodore 1660 modem
Commodore 1670 modem
VIC-1011a rs232a interface
VIC-1011b current loop interface
Datatronic Vic Rel relay interface
Full C64 Test/Diag Harnass
PET CB2 Sound Adapter
PET Joystick Adapter
SpeedDOS-type parallel cables for the 1541 (XP1541, XE1541, cables, etc..)
ComputerEyes video adapter
Video Byte II video digitizer
Aprotek Minimodem Cxx 1200 & 2400 baud modems 
Powerhouse X10 Home Automation system
VICcontroller BSR-type Home Automation system

Recent-er Peripherals
CMD geoCable for parallel/centronics printers
64NET Cable, for connecting C64 to PC
StrikeLink WiFi modem
Commodore4ever WiFi modem
WiModem WiFi modem
C64-Net WiFi modem
Comet 64 modem (original version)
GLINK-LT RS232 interface
TexElec SNES Controller Adapter
Stupid PET Tricks adapter
SAM Speech Module
ZOUNDS! Audio Digitizer
SUX 6400 Audio Digitizer
C64 4-player Joystick adapter