This was originally going to be about the Galidor Easter egg. I’m talking about this guy from Ninjago City(which I didn’t get but I wanted this figure):

Look, Galidor would have been a great theme were it just an action figure theme. However, as a LEGO theme, it’s not hard to seen why it’s considered the biggest failure among LEGO fans, and even by Mark Stafford. However, I do find it really cool that this minifigure torso actually had the easter egg. It’s the reason why I ordered one off of Bricklink.
However, I decided to have this topic be about LEGO Easter Eggs in general, be it a reference to another theme, another franchise, a person, etc.
Personally, I thought this was a nice touch on the SC Ferrari 312 T4:

Gilles Villeneuve was a renowned Formua One driver who actually drove a 312 T4. Tragically, he was killed in a crash in 1982, at the age of 32. I feel that whoever designed this set likely chose his 312 T4 as a tribute. They didn’t have to choose his car, but they did. It’s a small but meaningful tribute.
Comments
The more obscure Easter eggs tend to be things like designers initials and years of birth etc in vehicle registration plates.
https://www.bricklink.com/v2/catalog/catalogitem.page?P=2465pb14&idColor=85#T=C&C=85
The sticker actually reads 'Djordje was here' in dwarven runes (refering the line's graphical designer)
I also like that the hieroglyphs in the lower section of the Adventurer's sarcophagus spell out the word 'Lego' in real egypt hieroglyphs.
(https://www.bricklink.com/v2/catalog/catalogitem.page?P=30164px1&name=Container, Coffin Lid with Mummy Relief Colorful Pattern (Sarcophagus)&category=[Container, Decorated]#T=C&C=2)
The 1x2x5 Brick columns from the same theme also use authentic hieroglyphs and spell out the names of ancient egypt gods Anubis, Khnum, Thoth and Horus (although in an incorrect non-egyptian way)
Some of the runes printed on shields and tiles from the Vikings line also spell out 'Lego'.
https://www.reddit.com/r/lego/comments/5gvfau/exoforce_ninjago_crossover/davj4ui/
...the fact that this has an Aquaraiders helmet has rather dark implications for a LEGO set.
Light Bluish Grey. Was just going from memory before. You're welcome fellow Massachusettian. Wasn't aware we had a batcave here ;)
https://www.flickr.com/photos/mypeanut/14728349262