Beyoncé Giselle Knowles (Carter?)

Both an award-winning single-named singer and a talented actress, Beyonce has done relatively little genre work, but what she has done has been attention-grabbing!
Curiously, though her non-genre acting had been in "serious" films like Cadillac Records and DreamGirls, her genre movie work had only been in comedies...until Obsessed!

Genre appearances... 
Obsessed (Sharon Charles)

Austin Powers in Goldmember 
 (Foxxy Cleopatra, also performed song on soundtrack)
The Pink Panther
(Xania, also performed songs on soundtrack)
Pepsi Commercial "Dare for More: Gladiators"
(Gladiator, also performed song)

Check out...
The Official Beyonce HomePage
BeyonceWorld (FanPage)

Kerry Washington

Another actress (along with Jessica Alba) who's appeared in both Fantastic Four films...
Bounces between "serious" films like Rayand Last King of Scotland and "popcorn" flicks like Lakeview Terrace and I Think I Love My Wife with ease.

Genre appearances include...
Django Unchained (BroomHilda)
Scandal (Olivia Pope)
Black Panther (Princess Shuri aka Black Panther)
Lakeview Terrace (Lisa Mattson)

Fantastic Four / Fantastic Four: Rise of the Silver Surfer (Alicia Masters)
Mr & Mrs Smith (Jasmine)
Boston Legal (Chelina Hall)
Sin (Kassie)
WonderFalls (Mahandra McGinty)
[unaired pilot. Role assumed in series by Tracie Thoms]

Check out...
Kerry Washington Central (FanSite)

Other actresses to play Alicia Masters...
Fantastic Four: World's Greatest Heroes [2007] Sunita Prasad
Marvel Action Hour: Fantastic Four [1996] Paulina Lomas
The Fantastic Four [1994] Kat Green [live-action film, never officially released]
Fantastic Four: the VideoGame [2005] Cree Summer

Helen Rachel Schlacter

It's not often an actress can play a character, then play the character's aunt 24 years later.
Helen was the first actress to play Supergirl / Kara Zor-El in any medium in Supergirl: the Movie (1984)!
In 2008, she played Kara's aunt, Lara-El (birth mother of Kal-El / Clark Kent), in two episodes of Smallville! (She also appears in a documentary on the Season 7 Smallville DVD set entitled "Supergirl: Last Daughter of Krypton")
And she still looks...well...SUPER!
Currently a regular on the new TeenNick series Gigantic as Jennifer Moore.

Over three dozen film and credits...
genre appearances include:
Supernatural"Family Remains"
Eleventh Hour"Medea"
Smallville(Lara-El)
Steal (Kim)
Nowhere in Sight(Carly Bauer)
Michael Hayes (Julie Siegel)
House in the Hills(Alex Weaver)
Supergirl
(Kara Zor-El aka Linda Lee Danvers aka Supergirl)
Dream On(Sarah)
Batman: the Animated Series(Talia al Ghul)
ABC AfterSchool Specials "Amy & the Angel"
12:01(Lisa Fredricks)
Check out...
Helen Slater's Official Page
Tribute to Helen Slater

Other actresses to play Supergirl / Kara Zor-El / Linda Lee Danvers since Helen include
Nicolle Tom (Superman: the Animated Series, New Batman Adventures, Justice League Unlimited)
Laura Vandervoort (Smallville)
Tara Strong (Justice League Heroes video game)

Other actresses to play Lara-El aka Lara Lor-Van before Helen include...
Agnes Moorehead (Adventures of Superman [radio] "The Baby from Krypton" [1940], "Superman Comes to Earth" [1942])
Luana Walters (Superman[1948 movie serial] "Chapter 1: Superman Comes to Earth")
Aline Towne (Adventures of Superman "Superman on Earth")
Susannah York (Superman the Movie, Superman II, Superman IV: the Quest for Peace)
Britt Ekland (Adventures of Superboy "Escape to Earth" and "Abandon Earth")
Eliza Roberts (Lois & Clark: the New Adventures of Superman "Foundling")
Finola Hughes (Superman: the Animated Series "The Last Son of Krypton")
and, Kendall Cross on an earlier episode of Smallville ("Memoria")

Other actresses to play Talia al Ghul include...
Olivia Hussey (Superman: the Animated Series and Batman Beyond)
Andrea Bowen (Batman: the Brave and the Bold)