Wentworth Miller (Buffy Season 2, Episode 20, "Go Fish")
Miller is best known now for his starring role in "Prison Break", which ran for four seasons on FOX, but back in 1998, he was an unknown PA. He was cast as Sunnydale High swimmer Gage Petronzi in the Season 2 episode "Go Fish", which was his acting debut.
Josh Holloway (Angel Season 1, Episode 1, "City Of...")
When most people think about "Lost" stars who appeared on "Angel", the first name that comes to mind is Daniel Dae Kim, who played Jin on "Lost" and appeared in 12 episodes as Wolfram & Hart lawyer Gavin Park on "Angel". However, Kim was well-established (if not exactly well-known) by the time he appeared on "Angel". Not so for Holloway (best known as James "Sawyer" Ford), for whom the series premiere of "Angel" was his first acting role.
Christina Hendricks (Angel Season 1, Episode 15, "The Prodigal")
Hendricks can currently be seen as the lovely Joan Holloway on "Mad Men" and is even known among Joss Whedon fans for her two-episode stint as "Saffron" (not that we ever got the character's real name) on "Firefly". But before trying to con Malcolm Reynolds, Hendricks had a run in with a pre-vampire Angel and his sire Darla in a flashback on "Angel". It was her third-ever credited role.
Amy Adams (Buffy Season 5, Episode 6, "Family")
This real-life Disney princess actually had 8 credits to her name prior to appearing on "Buffy the Vampire Slayer", but it's safe to say she wasn't a star, otherwise she wouldn't have agreed to appear as Tara's cousin Beth in this Season 5 episode. Adams has one major scene opposite Amber Benson, but otherwise is a minor part of this episode. She's since gone on to much bigger things (Academy Award nomination, headlining major studio films)
Rachel Bilson (Buffy Season 7, Episode 18, "Dirty Girls")
Prior to appearing in this Season 7 episode as potential slayer Colleen, Rachel Bilson's only credit was as "Gum Chewing Girl" in an episode of "8 Simple Rules". It didn't take long for Bilson to make it big. Within a few months of "Dirty Girls" airing, "The O.C." debuted on FOX. Bilson would go on to appear in 92 episodes of the hit show, establishing her as a star. Considering she was on "Buffy" for about 2 minutes, I doubt this episode had any impact on her casting in "The O.C.".