|Charmed: The Wedding from Hell (1998) [Bridesmaid #1]: Destroyed (along with Sarah Peterson and the other demonic bridesmaids) by Todd Cattell. (Jennifer is wearing heavy demonic make-up for this role.) (Thanks to David31)
Summer of Sam (1999) [Valentina Suriani, Woman in Car]: Shot in the head by Michael Badalucco while sitting in a car with her boyfriend. Her body is also shown again afterwards when John Leguizamo arrives at the crime scene as the police investigate.
Angel: Blind Date (2001) [Vanessa Brewer]: Impaled with her own cane by David Boreanaz. (Thanks to Alex)
(Note: As a professional stunt performer, Jennifer has presumably "died" in place of several other actresses, though I don't know any specific examples.)