Greg responds...

I know this is in the archives... <sigh>

I won't pretend this is a complete list, but off the top of my head...

Classic Star Trek
Nichelle Nichols - Diane Maza

Next Generation
Jonathan Frakes - David Xanatos, Coyote (robot)
Marina Sirtis - Demona/Dominque Destine, Margot Yale
Brent Spiner - Puck
Michael Dorn - Coldstone, Taurus
LeVar Burton - Anansi

Deep Space Nine
Avery Brooks - Nokkar
Colm Meeney - Rory's father

Voyager
Kate Mulgrew - Anastasia/Titania

Plus there are all sorts of actors who had guest rolls on the various Trek series, for example both Morgan Shepard (Odin, King Kenneth, etc.) and Salli Richardson (Elisa/Delilah) have guested on Trek. I don't know and couldn't possibly list them all.

But this is a start, right?