Christopher Lloyd made an appearance in an episode, where there were some references made to Back to the Future. Fittingly enough, it was called Back to the Future IV: Judgement Day - and he appears as an old mentor of Mike's. Unfortunately, though, we don't get to hear him say "Great Scott!"
Meredith Baxter-Birney and Michael Gross had also made an appearance. Meredith was also Mike's mother, but Michael G played Mike's therapist.
Tracy Pollan made a couple of appearances, as well - as Mike's old flame. She, of course, appeared in in Family Ties before the two got serious - and then she also had a part in Bright Lights Big City.
Spin City was practically the Michael J Fox Reunion Show, until he had to leave after the forth season. I guess they also brought in important people from Charlie Sheen's life, but we won't get into that.
I haven't seen much Spin City myself, just that video that comprises MJF's favourite episodes. I really do want to see the episode that guest stars Christopher Lloyd, though.