Remote control vibrators. The sort that your partner can control with a phone app when they're not even in the same town.
I'm pretty sure Lovense work like that, but the cheaper brands I've found that are app controlled I'm not convinced can be controlled over the Internet.
And I've read stories about them being worn in public. Are they really quiet enough for that?
And can they be controlled while using other apps like voice calls or WhatsApp?