This this this!
I have a handy dandy list of crossovers and tie-ins I've been half-maintaining, and it's getting huge. They've really gone overboard.
Night of the Owls (12 Issues, 1 Annual, 1 Spinoff and one extra character)
Batman 8-9, Annual 1
Nightwing 8-9
All Star Western 9, Batman: The Dark Knight 9, Batwing 9, Batman & Robin 9, Birds of Prey 9, Catwoman 9, Detective Comics 9, Red Hood and the Outlaws 9
(Talon Spinoff)
(Strix in Batgirl 13 / Batgirl Annual 1 / BoP16)
Rise of the Vampires (5 Issues)
I Vampire 6 -> (End of JLD6) -> Justice League Dark 7 -> I Vampire 7 -> Justice League Dark 8 -> I Vampire 8
The Culling (6 Issues, 1 Annual, 1 Spinoff)
Prelude (Teen Titans / Superboy / Legion Lost 8)
Annual 1
Superboy 9 -> Legion Lost 9 -> Teen Titans 9
(Ravagers Spinoff)
Rotworld (13 Issues, 1 Annual [+ Possible zero issues, and possible Ravagersing])
Prelude - Animal Man 12 / Swamp Thing 12, Annual 1, (And the Zero Issues? I don't know)
Red Kingdom - Animal Man 13-16+ (Ravagers 5?)
Green Kingdom - Swamp Thing 13-16+
Secrets of the Dead - Frankenstein (0), 13-15
Rise of the Third Army (16 Issues, 1 Annual)
Green Lantern 13-16
Green Lantern Corps 13-16
Green Lantern: New Guardians 13-16
Red Lanterns 13-16
Green Lantern Corps Annual 1
Death of the Family (23 issues and still going, [+ TT14? and RHatO #0])
Batman 13-16+
Batman & Robin 14-16+
Batgirl 14-16+
Catwoman 13-14
Nightwing 14-16+
Suicide Squad 14-15
Teen Titans (14?) 15-16+
Red Hood and the Outlaws (0), 15-16+
Detective Comics 15-16+
H'el on Earth (10 Issues [and ongoing?], 1 Annual)
Superman 13 -> Superboy 14 -> Supergirl 14 -> Superman 14
Superman / Supergirl / Superboy 15
Superman / Supergirl / Superboy 16 (Superboy Annual 1?)
Eclipso / Black Diamond (9 Issues, presumably more to come)
Demon Knights 14-15
All Star Western 14-15
Team 7 #2-4
Catwoman 15-16
Throne of Atlantis (5 Issues?)
Justice League 15 -> Aquaman 15 -> Justice League 16 -> Aquaman 16 -> ? (It says 16 is the penultimate chapter, but I don't know if it would finish in JL 17, Aquaman 17, or an annual tbh)
Mini Crossovers
Suicide Squad/ Resurrection Man 9
Blue Beetle / GL:NG 9
Firestorm / JLI 9
Teen Titans / DC Universe Presents 12
Superboy 13 / Ravagers 5? (This one is only a possible - it sounded like a mini-crossover, but might just be cameo time)
O.M.A.C / Frankenstein 5 (which was a "two sides of the same story" deal)