Warning: Major spoilers ahead!
1. The death of Adrian Mellon — the man who’s beaten and thrown over the bridge by homophobic Derry locals — is meant to show Pennywise’s pure evil influence on others (in this case the Derry locals) even while he is hibernating.
2. The person directing Bill's movie The Attic Room is Peter Bogdanovich (presumably just playing himself, since he's called "Peter").
3. And The Attic Room is being filmed on a Warner Bros. stage — the studio behind It Chapter Two.
4. Adult Eddie's wife Myra is played by the same actor who plays his mom as well, Molly Atkinson.
5. One of the people working for adult Ben in the boardroom scene is actor Brandon Crane, who played young Ben Hanscom in the 1990 miniseries.
6. Stanley is working on a bird jigsaw puzzle when he gets the call from Mike — birds are very connected to Mike in the book.
7. And birds appear again during Mike's flashback where he learned about Its true form and arrival billions of years ago.
8. Once the adult Losers arrive in Derry, they never change their clothes.
9. The underside of Dean's (the kid Bill tries to save) skateboard has the same iconic pattern as the carpet from the Overlook Hotel in Stanley Kubrick's adaptation of The Shining.
10. The bar/lounge inside the Derry Townhouse has a nautical theme and is decorated with a lot of miniature sailboats — a nod to Georgie's paper boat.
11. There's an old Pennywise the Dancing Clown poster in the background of Mike's attic apartment.
12. Mike also has a picture of Derry's famous Standpipe on his table — it's the same one that was featured on the postcard Ben gave Beverly in the first movie.
13. In the flashbacks at the clubhouse, Eddie has a T-shirt with an evil car on it, a reference to King's book Christine.
14. And Christine’s license plate (CQB 241) from Carpenter's adaptation hangs above the counter in the Secondhand Rose thrift shop.
15. During Ben's flashback at school, you can see a turtle on the teacher's desk (the image below is of a Lego turtle from Chapter One).
16. And when Pennywise, as Beverly, says, "Kiss me fatboy!" That's a reference to the 1990 TV miniseries.
17. The film's director, Andy Muschietti, makes a brief cameo in the pharmacy during Eddie's flashback.
18. In the newspaper clipping of the fire that Mike's parents died in, the words "IT" can be seen in the smoke above the building.
19. In the funhouse, the swinging clown puppets resemble Pennywise from the original It TV miniseries.
20. You can see the words: "Big is what it seems" when Beverly is stuck inside the bathroom stall during the battle with It...
21. ...the words, "Hate Clown" can also be seen.
22. You hear Henry Bowers yell, "Heeeere's Johnny" when Beverly is fighting to keep the stall door closed — another nod to The Shining.
23. The scary legs that dance towards Richie and Eddie are Betty Ripson's legs. (One of the teens who was killed by Pennywise in Chapter One.)
24. When Pennywise reveals Mike lied to the group, Richie yells, "Are you fucking kidding me, Mike?!" — That's Bill Hader's personal in-joke reference to John Carpenter's adaptation of The Thing.
25. When It is dying, one of its eyes is blue, and the other is yellow.
26. Bill has Mike's cell phone number actally saved under his name at the end of the movie.
27. And finally the book on Mike's desk at the end of the movie, A History of Derry, is written by Branson Buddinger, which is an actual character in the novel.
