Here's a spreadsheet with a list of Sega Dreamcast online titles, including the features they include. The list isn't 100% complete, and I'll fill it in as I learn new information and correct any mistakes. It includes the Japanese titles too, giving us a solid list of 184 online DC titles!
Considering it's very short 2 year lifespan, that's a HUGE number of online-capable titles. Not all of these included online play. Several simply include an accompanying Homepage, most allow for additional DLC and/or Leaderboards and a few (mostly Capcom titles) include additional online play. I've also listed which regions are included, whether the game is VGA compatible and the status of each the web-side of each title. Some sites are listed as Awaiting Upload, this is because the site itself has been completed and is simply awaiting final touches and htaccess before being uploaded to Dreampipe (thereby making it accessible via the in-game browsers), or they may be awaiting redirects by Kazade to point the Dreamcast browser at the right site URL.
I hope this is of interest to someone, and any missing gaps of information that you know the answer to please reply here and I'll update the file
EDIT: Just out of interest, there are 54 online multiplayer titles (Not including the ones with ? Players listed as I'm uncertain of their online status)