Gravity Falls in Rick and Morty
In the episode Close Rick-Counters of the Rick Kind, Rick opens up a bunch of portals to other dimensions. One of the portals shows a mug with a question mark on it, a pen, and a notepad come out of it. This is a continuity of the Gravity Falls episode Society of the Blind Eye, where the episode's ending credits gag features Stan losing those three exact things, by having them get sucked into a Universe Portal.
In the episode Big Trouble In Little Sanchez, a picture of what appears to be a triangle with one eye and a top hat, which resembles the Gravity Falls character Bill Cipher can be seen on a monitor.