The mid-late 90s personal organizer. The big companies making them were Sharp and Casio, although there were a lot of other ones.

I still have mine, the IR-7000, made by Sega. Looks like this:

Still works turns out.

Like the standard organizer it flipped open and had a QWERTY keyboard. It could store notes, addresses, do calculator functions, time zone calculations, set an alarm and show you a calendar. Plus, a hilarious "human portrait" maker along with a simple game you can play with the portraits.

If two people had IR-7000, you could use its infra-red communication to exchange messages, but I never came across someone who also had this organizer. The industry was really fragmented toward lots of different organizers and everyone seemed to have a different one.

The modern equivalent today would be something between a cellphone or tablet. Cellphone and tablet subsume all of the functionality that these organizers had, but in much more general-purpose ways with fuller software stacks. I can understand why these fuller software stacks are desirable yet in my heart I'm always keeping a space for the long battery life and reliability of this specialized tool for specific things.