Assembler Games user Dreamcast recently posted their work in progress, the Dreamcast Moband, a Modem to Broadband replacement adapter. Essentially, once finished, it will allow a standard DC to communicate via ethernet cables without needing one of those Broadband Adapters that still go for hundreds of dollars. Now DC homebrew developers will have a new tool to help develop online games or functionality for the Dreamcast.

Dreamcast moband

You can learn more about it by checking out this thread on Assembler Games.