David Boreanaz sent on to star in a TV show called Bones after Angel finished.
K.D. Aubert, who played Nikki Wood in Lies My Parents Told Me, played a character called Toni in an episode of Bones called ‘The Man In The Bear’.
Leonard Roberts who played Forrest Gates in season 4 of Buffy appeared in an episode of Bones called ‘The Girl In The Fridge’ as a DA named Andrew Levitt.
Harry Groener, who played The Mayor in season 3, also appeared in an episode of Bones called ‘The Woman At The Airport’ as a plastic surgeon called Dr Henry Atlas.
There was also a reference to Joss Whedon’s Firefly in Bones… in the episode ‘The Man In The Fallout Shelter’, Hodgins says to Zach, “When you ask it to take out the trash, it sits on the couch watching re-runs of Firefly”.
In Bones, Seeley Booth, played by David Boreanaz, used to be a military sniper and he now wants to catch murderers to make up for all the lives he took.

Seeley: “Me being a sniper, I took a lot of lives. What I would like to do before I’m done, is try to catch at least that many murderers.”

This is kind of like the character of Angel, who wants atonement for all the people he murdered when he was Angelus so he now saves peoples lives.