Benj Edwards of Vintage Computing and Gaming has written a good piece on modems for PC World. Today's cable modems are a largely overlooked piece of equipment, just another appliance that helps get things done every day; but in the 80s and 90s it was a huge kick to upgrade your modem. If you were using online systems then you surely remember the boost in speed from 300 baud to 1200, and even larger leaps in later years. My first was a Smartmodem 1200, by Hayes, a device that improved the speed of the revolutionary original Smartmodem. The Smartmodem 300 obviated the need for connecting to the phone line via an acoustic coupler and made a quantum leap for the industry by introducing the Hayes command set: a series of codes that could be embedded in the data stream itself - that is, you could send commands to your modem (or a remote modem) just by typing them in using your communication program on your computer.
Here's a review of the Smartmodem 1200 from a 1983 issue of Infoworld. The original Smartmodem came out in July 1981, a month before the IBM PC. The Smartmodem 1200 followed the next year. Pictured: the 9600 model, from 1988.