There are a lot of easter eggs strewn about in the original portal. My personal favorites were the pictures of jellyfishes scribbled on some of the walls in the later levels. I was curious to know which ones you found and which are your favorite?

I'd like to see a list of everything like that in the game. I've played through it two and half times and I still feel like I could find something brand new next time round.

I still think the singing Turret room is my favorite easter egg, probably because I wasn't expecting it and just happened upon it by chance.
 
Close second is the Ratman's last transmission that turned out to be a picture of the Companion Cube on the moon.

@LordXavierBritish said:

I still think the singing Turret room is my favorite easter egg, probably because I wasn't expecting it and just happened upon it by chance. Close second is the Ratman's last transmission that turned out to be a picture of the Companion Cube on the moon.

I haven't seen these both, just found some funny wall paintings on some random secret locations, seems I need to play it again carefully and find out all the radios :P