Here is a list of Dreamcast games still working online. I got this from the GameFAQs Dreamcast forums.

US and EU Games Online through official servers:

4x4 Evo (both full and demo versions)
Maximum Pool (requires using this game save: http://www.pauth.com/download/dc_max_saves/MAX2.VMI )
Sega Swirl (requires a fully operational POP mail account)
Starlancer
Phantasy Star Online (European Version)
Phantasy Star Online Version 2 (European Version)


JP Games Online through official servers:
Note: Japanese games cost a fee to play online.

Phantasy Star Online
Phantasy Star Online Version 2


Games online through private servers:

Quake 3: Arena (various privately hosted servers)
Phantasy Star Online (All Versions) ( http://pages.globetrotter.net/ctrep/ )
Phantasy Star Online Version 2 (All Versions) ( http://pages.globetrotter.net/ctrep/ )

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I've also come across this guide on how to connect a Dreamcast to a broadband Internet connection using its built in modem.

This method involves you connecting the DC to a PC with a modem and then sharing the PCs internet connection with the DC.

The full guide can be found at: http://www.consolevision.com/members/mterlouw/
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Excellent ive always wanted to get the dreamcast online but never had the opportunity, cant wait to check that out
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Never hooked the DC to a network, think that's more something for gamers, and not collectors.
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Yeah might have changed my mind on that one looks a bit complicated
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The guide does look a bit complicated when first looking at it, but basically all you are doing is tricking the PC to receive the incoming call from the DC and pass the broadband connection on to it. I had a quick read through and it isn't all the complex in reality.

In the Dreamcast days it was quite amazing to be able to connect it to the internet. It's browser was pretty good too. I think Dreamkey 3.1 is still quite compatible with today's websites. I've not tried it since I stopped using dial-up internet though. I may give the guide a go and post back if it works.
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yeah i really should try it i have Dreamkey 3.1 and probably have the neccesary cables too (frickin cables everywhere, throw sumat out keep the cables?) and ive got quake 3 i think too maybe i could blag onto a private server and get shot for fun
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Cables do seem to breed somehow! I've got boxes and boxes of cables on top of a cupboard and when I need one I always wonder why I have half of them in the boxes as I will never probably use them, and yet I still keep them.
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