NullDC would be the best offering. The latest stable release was made about a year ago, however it still is in the works. While many games work, there certianly are some issues to considder.
However the nice thing is is that NullDC has both a Dreamcast and NAOMI version, though the NAOMI version is a bit lacking in support and not every NAOMI game has been dumped. Personally I'm very eager to see if Sega Strike Fighter will run well if it ever gets dumped. I've been hunting for a good arcade flight combat game for my gaming events.