Bitworld.

Here's a FREE game for you. WW2 - The Big One
This is a pbem recreation of the Second World War in great detail. There are Strategic, Operational, and Tactical levels. Teams of players battle on land, sea, and in the air.
There are NO DIE used and no die roll simulator. The game is free to play and the rules are a free download.
This game may well be the biggest one you will ever play. It is for thinkers and requires a lot of email communications. Are you up for a challenge?
http:/ww2thebigone.webs.com

Jesse Venbrux is pretty prolific, and has done a couple of incredible games. In particular I recommend Frozzd and Karoshi 2.0, though everything else has its own merits.
And then there's and [url=http://www.auntiepixelante.com/]Antie Pixelante, whose games defy conventional description.
Finally you can always get a few hours of enjoyment from the results of Ludum Dare, a 48-hour game-making competition.

The Spirit Engine 2 is going freeware, though the developer is still looking for host...

by Pixel (Cave story). [url=http://www.youtube.com/watch?v=InfFJTVAMkw]Ikachan gameplay.

No thanks.
What is it with people and childish MMOs?

This is a little debatable if it is a game or not, but Super Columbine Massacre RPG is a game made with RPG Maker based on the Columbine Massacre. The game is played through the eyes of the 2 troubled teen shooters, and from what I understand uses actual audio and pictures of the events. I think it is just a work of art and an interactive story telling of the game. I'm fixing to play it myself.

Okay, I'm a little confused. Do you make maps for the game? I need some help here. =)
That sounds fun.

I've found two giant lists of free games:
365 days of free games
30 commercial games released for FREE

Adding from this thread:
Sid Meier's Railroad Tycoon

Fallout freeware ? It's on GoG's catalog and you can get a retail version in store (Fallout 1&2 + Fallout tactics).

I know, currently I own F1, F2 and Tactics here from GOG. ;)
I've never used Gametap, and disliked that they promoted the service on that list. Since it's a company holding the software and you need to log in to get it, it isn't freeware really.
EDIT: But at the same time, they added a link for GOG. At least for BaSS they did.

It's 'free to play', not freeware. Same with flashpoint. Gametap is more of a digital game rental service than digital game store.

You're right, free to play. That's why I said they promoted the service, which isn't really a distribution of freeware games, but distribution of commercial games under a "free" label.
PS: I feel pity for steam users who still buy BattleForge for $29.99. A big LOL for them.

For some reason all free games lists everywhere are missing Eye of the Kraken, which is a very nice adventure game (at least for a free download).
As fan games get listed too, then a formerly commercial game like this should be on those lists too. Not that it's the best game ever, but not having it on any list is way too much underrating for it, because it's quite fun to play for adventure fans.