This is a list of Sabrina's known guest appearances in other TV shows.  If you are aware of any more, however small the part, then let me know.
 
Superboy (1988)
Episode: "Bringing Down the House"
Character: Betsy
 
This may well represent Sabrina's earliest screen appearance - she would have been only 17 or 18 at the time. She plays a friend of Lana Lane who, along with lana gets all excited about the return to town of a local rock star (played by Lief Garret - remember him?). Suffice to say that he turns out to be not such a nice guy as everyone thinks and Superboy sorts him out (he's not exactly an arch-criminal though - I think Superboy could have left it to the cops). Sabrina appears in a few scenes, and has a few lines. She seems to be Lana's best friend in this, so I'm surprised her role didn't recur.
NB: The first season of Superboy, containing this episode, is currently available on dvd.
 
New Leave it to Beaver (c1988)
Episode: Not known
Character: Not known
 
I can't confirm this appearance 100% but I was told about it by an apparently reputable source.  Sabrina played a hitch-hiker that is picked up by "the boys" (never having seen the show I don't know who "the boys" are).  This is a mistake on their part because she's a bad girl. She produces a gun, makes them strip to their shorts and steals their truck.  Sounds cool :)
 
As I say, I can't verify this but it sounds plausible enough. There's probably a number of little guest slots she did before getting bigger roles.  Anyway, I suppose it's possible that the show gets repeated somewhere from time to time so keep your eyes peeled. Unfortunately I have no idea which episode, or even which season, she was in.
 
Law & Order (1992)
Episode: "Intolerance"
Character: Kate
 
Sabrina plays Kate, the girlfriend of a boy who is shot in what is first assumed to be a Chinese gang-related killing. The motive turns out to be something very different - academic jealousy and maternal pride. Kate knows the truth but there is no evidence. So she lies on the witness stand to implicate the murderers. Lots of nice emotional scenes in this episode for Sabrina to display her talents.
 
NB: The second season of Law & Order, containing this episode, is currently available on dvd.
 
 
Untied - Superchunk (1992)
Sabrina appears along with the band and Santo Fazio in this music video. Interesting if you want to see Sabrina put live fireworks in her mouth.
 
The "No Alternative" Video (1994)
Video: "Iris" by The Breeders
 
Sabrina appears alongside Parker Posey in the music video for this track by The Breeders which is part of a compilation put together in aid of AIDS education.  Nothing much more to say except that she looks fantastic.  
 
 
3rd Rock From the Sun (2000)
Episode: "Dick'll Take Manhattan"
Character: Not named
 
This was a RUMOURED guest appearance for Sabrina which turned out NOT to be true.  I don't know who the actress is but she DOES bear a resemblence to Sabrina.  Sadly it wasn't her, which is a pity because it would have been very approriate since this episode features the aliens getting bored with their lives and travelling to an alternate Earth to see if they are doing any better there.  Sabrina, of course, knows all about alternate Earths.