Almost as important as the modem was the transfer protocols for BBSes. While Zmodem was generally king of the hill:

* https://en.wikipedia.org/wiki/ZMODEM

Later BiModem and especially HS/Link built on top of that:

* https://en.wikipedia.org/wiki/BiModem

* https://en.wikipedia.org/wiki/HS/Link

Being able to download and upload files at the same time really helped make things more time efficient. From downloading files from BBS door games (e.g., Trade Wars 2002):

* https://en.wikipedia.org/wiki/BBS_door

To one's mail and forum postings via QWK (and SOUP for offline Usenet reading):

* https://en.wikipedia.org/wiki/QWK_(file_format)