What are the must-play Freeware/Browser-Based PC games?

Inspired by this thread.

Having immersed myself in the medium* lately, here’s what I can remember:

2-D Platformer
Super Mario 63

You may be rolling your eyes but this game…is like the framework of Super Mario World, mixed with the physics and plot of Super Mario 64, the graphics of Yoshi’s Island, and the music of Super Mario Galaxy.

Adventure
Cave Story

Tower Defense
Bloons Tower Defense series (with 3 being the pinnacle)

RPGlike
Sonny

Puzzle
…there are so many, it’s hard to pick just one. Really, free online games represent the triumph of the Puzzle genre.
A few: Loops of Zen, Factory Balls series, Shift series

Shoot-em Up
Frantic 2

Shooter (3rd person)
Iji

MMORPG
Kingdom of Loathing

Gimmick
Upgrade Complete!

*Have these games even been around in force for 10 years yet?

Freeware games have been around forever.

Roguelike: Nethack

Interactive Fiction. Hrm. Maybe Spider and Web. Or you could go old-school and cite Colossal Cave Adventure.

For Tower Defense I kind of like Desktop TD Pro, myself.

Fantastic Contraption is a great puzzle game.

For interaction fiction, I’d have to say Photopia, though, it’s not really a “game” in any sort of traditional IF sense.

Dolphin Olympics seems to fit in this category more than any others, and is a million times awesomer than Upgrade Complete!

Really, how about including a link or two? I don’t imagine googling Shift is going to give me a whole lot of enjoyment.

Here’s a link to Mario 63.

http://www.sheezyart.com/art/view/1616035/