1. Firefox: And, Lo! The Book of Mozilla
Firefox is easily the best browser to hunt for Easter Eggs. This one, "about:mozilla", started as a response to a now defunct special page in Explorer 5.5 that showed the blue screen of death when you typed "about:mozilla." Bonus: Typing "res://mshtml.dll/about.moz" in Explorer will show the old blue screen.
2. Firefox: Klaatu barada nikto!
Experience multiple levels of geek with Firefox's "about:robots". Bonus: Please do not press the button.
3. Firefox: I herd you like browsers...
Get a browser in yo browser with "chrome://browser/content/browser.xul". This works multiple times, so you can really go the full Inception on this. Bonus: Originally, there was a Ghostbusters nod with a secret link to Mozilla's now defunct XUL project.
4. Chrome: Aw, Snap!
While there are several special pages to geek out over in Chrome, (about:memory, about:stats, about:histograms) this one, about:crash, gets Chrome to show the sad folder logo.
5. Chrome: It's a series of tubes!
This one is a little special case. If you have the 'pipes' screen saver installed, when you type "about:internets" into your browser, you get to see what Senator Ted Stevens saw in his browser.
6. Opera: /.
Typing "/." into Opera will take you directly to slashdot.org
7. Netscape: Book of Mozilla Redux
If there are any dinosaurs out there, typing "about.mozilla" into Firefox's elderly grandfather, Netscape, will get you another poignant quote from the Book of Mozilla.
8. Bonus iPhone Easter Egg
iPhone users: Google "tilt" in mobile Safari and see what happens.