The Easter egg as a concept can be dated back to rich aristocrats hiding things inside priceless jewellry. But in the modern age, you're far more likely to encounter one when you Konami-code a website, or made some odd menu choices in a game.
Personally, I find it difficult to decide on my favourite. Google has a strong heritage with dicking around — do a barrel roll, the infamous swimming directions, and the YouTube loading animation being class-A examples. But honestly, the prize has to go to Excel 97's hidden flight simulator. Anything that turns spreadsheet software into a game is already pretty good, but the convoluted path you had to take to get there made this virtually unbeatable*.
With the whole wide internet out there, though, I'm willing to bet that you've seen some things. Post your best (with URL, you tease) down below.
*I appreciate that this isn't exactly on the Internet. Shush.