It isn’t just birds that lay eggs, in fact there are far more egg-laying species than there are placental mammals. So naturally the eggs vary a lot and can be quite weird …
Birds
There are over 50 breeds of chicken and the colour of their eggs is dictated by genetics. All eggs start out white and any pigment is deposited during the egg’s 26-hour journey through the hen’s oviduct. What’s more, you can often predict what colour a chicken egg will be by the hen’s earlobes. (Bet you didn’t know hens even had earlobes!)

Leghorn chickens lay white eggs and have white earlobes, while chickens with red earlobes lay brown-shelled eggs — but of course there are exceptions. Araucanas lay blue-shelled eggs and when you cross them with a breed that has eggs of a different colour the dominant blue-shell gene makes the resulting eggs blue, pink or even green.
The tinamou (an ostrich relative) may have evolved bright turquoise eggs to attract other females and encourage them to lay their eggs nearby, creating a sort of safety-in-numbers strategy for avoiding predators. Curiously the tinamou’s eggs are also as shiny as Christmas ornaments.
And cassowaries have bright, almost fluorescent, green eggs.
Insects
The green lacewing creates a silky stalk on which it hangs its eggs. This keeps the lacewing larvae safe from predators — and cannibals. The insect also coats the stalks with a chemical defence against ants.
Many butterflies and moths lay beautifully sculpted and shaped eggs, like this owl butterfly egg …

Sharks
The horn shark has a spiral-shaped egg which looks like a natural drill bit and allows the mother shark to screw the egg case into hard crevices making it tough for predators to get them.

The egg cases of other sharks and rays — often called mermaid’s purses — come in a variety of shapes from sculpted flatfish-like to ravioli shapes. These egg cases are not like birds’ eggs in that the case is porous with both water and oxygen able to flow through to the growing embryo.
Based on Weird Animal Question of the Week: Oddest Eggs of the Animal Kingdom.