I'm not sure about ones like the VIA, ACIA, etc.; but I've used various smaller ICs and the intelligent character LCDs that have enable and register-select pins in addition to data, so I suppose those would qualify as you can put them on a processor's bus assuming you have the right address-decoding scheme and the IC will operate at the needed speed. One I used was the Saronix RTC58321 RTC (real-time clock, which keeps time of day and the date). It had a 4-bit data bus, so when I put it on the 65c02 bus, I just didn't pay attention to the upper 4 bits. It was also very slow, but that wasn't a problem in this case since the processor usually ran at only 150kHz to save power, and the whole computer, including the display, took only 2mA. That one had a 1-line by 16-character intelligent character LCD also, directly on the bus. Similarly, you could put various A/D converters, 74-family latches (for output) and buffers (for input), etc. on the bus, which I did on
my first home-made computer. I think all the bigger I/O ICs I've seen were oriented toward specific buses, whether 6502, Z80, 68K, or other bus.
