I clicked on the system button and then on the Atari-Link to have a look at some Atari-Games. But instead I got the list of -> C64 games. Same problem with C64->SNES, Gameboy->PC, NES->Atari, PC->Gameboy, SNES->Vectrex,
Vectrex->ZX Spectrum and ZX Spectrum->NES
Only the Amiga- and the Plus/4-Link are working properly.

Uh... I must have broken this when the NES category was established, i.e. a looooong time ago. You're apparantely the first one to notice, even I didn't think anyone was still using that way to navigate the site. Anyway, it's fixed now. Thanks for reporting.