The vast majority of minidisc players were also recorders, something that never became the norm on MP3 players - though iRiver's H series had mic and line in inputs, and iPods were well supported by 3rd party microphones through the 13 pin socket. It's also possible to solder an audio input onto a SanDisk Clip in place of its integrated microphone (designed for voice memos).
Until recently, Sony phones boasted an extra ring on on their TRRS 3.5 mm socket so they could support stereo mic input at the same time as stereo output - handy for phone-based active noise cancellation as well as audio capture on a budget.