Here are links to all of the official websites for cast members from the various series that I could find. I have not included links to official sites if they haven't been updated for at least 2 years or if there is no content at the site.

William Shatner
Leonard Nimoy Photography
Walter Koenig
George Takei

Patrick Stewart
Brent Spiner
Marina Sirtis
Wil Wheaton (blog)
Denise Crosby

Alexander Siddig
Nana Visitor
Chase Masterson

Kate Mulgrew
Roxann Dawson
Robert Duncan McNeill
Robert Picardo
Tim Russ

Dominic Keating
Anthony Montgomery
John Billingsley

If I have missed an official site for any cast member, please PM me and I will add it to the list as long as it is a current site.