Sonic the Hedgehog is a popular character originating the eponymous Sega video game series. He also had a long-running comic series by Archie Comics that had been published for 24 years before it finally ended. Sonic's comics were subsequently picked up by IDW Publishing.
While the Teenage Mutant Ninja Turtles Adventures comics were still in print, the Turtles made a cameo appearance in Sonic the Hedgehog #2 miniseries within a panel where Michaelangelo tries to make a guest appearance and help out Sonic's new comic book only to be told to scram.
In Sonic the Hedgehog #10, all four Turtles were seen in the background during Sonic's trek through the sewers, the brothers comment that they're in the wrong comic book.
In the regular series of Knuckles the Echidna #3, when Knuckles' home Angel Island is on fire, a naked Michaelangelo can be seen making a cameo amongst the fleeing animals.
In Sonic Universe #26, when Silver the Hedgehog travels to an alternate timeline, he and the Dark Freedom Fighters are seen using a base that resembles the Turtle Lair.
Later on in Sonic Universe #29, Bebop and Rocksteady make a cameo appearance as inmates of the inter-dimensional Zone Jail teasing Sonic's evil doppelgänger, Scourge.
Finally in Sonic Boom #3, after Amy loses her Piko Hammer she is seen trying out various other weapons, including a Bō that looked similar to the one wielded by Donatello.