Shamelessly stolen from a thread on /a/, I give you some homages to western media in anime and vice versa.
Little Witch Academia apparently features several characters from western cartoons in some of its background shots. Supposedly the top characters in the red circle are Mabel and Dipper from Gravity Falls and the one circled in the lower left is a character from Steven Universe. I haven't watched either of those shows, so I can't really comment myself, but it's not too surprising to me, seeing as Studio Trigger love to reference western stuff in their productions.
In the anime "The Unlimited Hyobu Kyosuke" both the Nostalgia Critic and the Angry Video Game Nerd show up as background characters... Seriously.
Pani Poni (?) has a pretty unsubtle reference to Back to the Future
Stan Lee shows up in the anime "Heroman", which I suppose is not entirely unsurprising
Kill la Kill has a couple of familiar faces in its background. Trigger does it again xD
Punchline has this frame in it, which features Robert Downey Jr. (I think), Samuel L. Jackson, John Travolta, Harrison Ford and Arnold Schwarzenegger
Kuromokuro (I think it is anyway) has a cameo appearance of Agent Mulder and Agent Scully
Sabagebu! has a very obvious reference to Mad Max
Panty and Stocking with Garterbelt has so many references to western culture, media and animation that there is not much point in trying to list all of it in this post, but this one in particular is pretty obvious.
The Monogatari series also feature some nods to western animation. Like the Panty and Stocking one, I feel that this one is obvious
The Monogatari series also has a pretty obvious Shawshank Redemption reference in it
A Japanese voice actress by the name of "Izumi Kitta" does the voice of Tomoko in the anime adaptation of the WataMote manga. Guess what character she also voices in Japanese.
And finally we have something in reverse. A reference to Japanese animation in a western cartoon. Notice something on the board in the background? Apparently Pinky and the Brain has a Neon Genesis Evangelion reference in it.