Nathan Fillion played Malcolm Reynolds on Firefly and also Caleb the priest on Buffy.
David Boreanaz played the vampire Angel on Buffy and then on the spin-off Angel.
Gina Torres played Jasmine on Angel as well as Zoë Washburne on Firefly.
Alan Tudyk played Hoban Washburne on Firefly and appears in Dollhouse as Stephen Kepler.
Eliza Dushku plays Echo on Dollhouse and also rogue slayer Faith on Buffy.
Charisma Carpenter played Cordelia Chase on Buffy as well as on Angel.
Amy Acker played Winnifred Burkle on Angel, then Dr Claire Saunders on Dollhouse.
It just goes on like this.
Mokalus of Borg
PS - I'm not sure there's anyone who's been in every one of Joss Whedon's shows.
PPS - I tried to stick with characters who actually encountered each other.