Basically what every small retail business is using: https://squareup.com/pos
There are others out there, but Square still seems to dominate.
From a consumer standpoint, this is an excellent option for a small business. As an example, a contractor I used for a home remodel project, started using this in conjunction with his cell phone. Plugged a device into the phone, I swiped my card, signed the appropriate box, was approved for him and then I immediately got a notification from my card issuer of the charge, via my cell phone email notification. A win-win situation for both of us.
If you consider something of this nature, your customers will likely appreciate it!