There is no good Dreamcast emulator for the Mac yet. Still LXDream, a Linux based Dreamcast emulator is working on OSX support (probably intel macs only though). If oyu have an intel Mac, you could consider booting up in XP.
A nice blog with lots of things about the Dreamscast is the Dreamcast Junkyard
My personal favourite games are.
* Samba de Amigo
Rhytm based and lovely
* Crazy Taxi
Great Arcade port of taxi simulator
* Gigawing, important shoot'em up that redressed the balance between player and enemy. See here for more details
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* Densha de Go, Arcade train simulator
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