Abandonware is NOT freeware, just wanted to mention that ;-)
God of thunder and Jetpack must be my favourite old games that turned freeware. They're just so extremely addicting.
Then there also Bio Menace, a classic from Apogee.
Beneath a steel sky is nice too.

IVAN - a Roguelike with nice use of tile graphics, like 'ambient' lighting, visual depiction of player and NPCs appearance, health and status, body fluids, limb severing and decapitation, stuff like that. it is also extremely unforgiving. drawbacks are not quite as many player actions as NetHack, and is only available for DOS and Windows. this is one game i've played more than any other in the past few years...
see attached screenshots for my current games (which i'm sure will be over very soon due to some ridiculous death)

That's why I love rogue-like. Stressful, though :P

Understatement? IIRC at one point the devs were treating victory reports as bug reports.
Roguelikes are great. Dungeon Crawl is the best modern one, in my opinion, and Dwarf Fortress has a roguelike mode, although most people play for the fortress mode, and even more find they get enough entertainment simply from reading the dev notes. Unreal World is a good one, although it's commercial - it's a "survive in finland" simulator. You can build stuff. Lots of the newer roguelikes are fun - DooMRL somehow manages to feel just like the FPS, despite being text-based. CastlevaniaRL is fun, but I never really got into it.
I've missed hundreds of great roguelikes, but I can't be bothered to name every one I've enjoyed. Incursion was neat and very D20, there was a recent one where you played a mage combining items to produce weird effects, Angband still has far too many variants, GearHead has giant robots...

yup, i died stupidly. why did i decide to wander the mine field testing area with a backpack of gunpowder? even with an amulet of life-saving... i died twice from the explosions. and in the other game, while scavenging loot from the level, i was just pressing down one direction key to get to the dungeon shop, and basically committed suicide by colliding with a werewolf, who was much more powerful than i. sooo dumb

For complete freeware games I have liked FreeCiv and OpenTTD greatly. Really entertaining game is Wolfenstain: Enemy Territory. I'm also big fan of rogue like games, Dungeon Crawl and Adom being most played.
There is tons of other freeware games too. It is quite easy to dig into those with linux, your package manager have nice amount of games few clicks away. I have played mostly with linux, so game either has to work natively there, or then work well with wine.
Shame how often smaller freeware game can beat big commercial game in every other aspect except visuals. Big budget games should have more content, and less visuals, and they would be played more. Back in old days, like what this site games are, things were different. :)

and it seems that everyone is knowing that excebt the people making games

Must be different technique how games are done. Commercial games have closed planning, exact time when it should be done and ready, and still it should make profit or at least pay itself back :)
Same thing with Hollywood movies. :)
Freeware games are started with little thing, released first alpha or beta, and people start commenting on them right away, then game is developed basing those comments. And they can focus on smaller group of peoples, and no need to get such provit (most of the time).
However, good old times games were better than these days... :P

bump-o for newb-o

So many post in and nobody has mentioned Battle for Wesnoth yet? It truly is one of the best turn-based strategy games made and perhaps the best freeware game ever. It really has a commercial quality to it.
But when Dwarf Fortress is finished, that will take the crown of best freeware game and perhaps the best game. The complexity the game already has and the amount of complexity it will get is something never seen before in the games. I just hope, no, pray they create a graphical shell for it. I know, ASCII is cool and hardcore and all, but some nice simple graphics (a la Civilization 1 perhaps?) would be better.

To those of you who like roguelikes with some actual graphics rather than ASCII, I would like to recommend
Vulture's Eye and Vulture's Claw; one uses the NetHack core and the other uses the SlashEM core.
Download page for both.

i was going to mention ur-quan masters but someone beat me to it (i guess more abandonware than freeware), one of the greatest games ever. Might require a bit of patience for newer gamers as the beginning doesn't go that quickly.

Two more freebies:
MechCommander 1 and 2

For anyone who has many fond memories of the original Xcom, I can't recommend UFO : Alien Invasion enough. It has nothing to do with the commercial UFO series (Aftermath, Afterlight, Extraterrestrials, etc), by the way. Has a nice look to it, they have the UFO interceptions in the game now, and the character and weapons selection and development are fantastic (for example...you can buy the specs to a gun, then have your engineers improve it, or develop better ammo for it).
http://ufoai.sourceforge.net/

Wesnoth ... It was really good - tons of units, tactical options... But it's 'battle system' was SOOO SCREWED UP! Elite units with 50% chance to hit? Come on, it was more of a loading festival - it was pretty much unbeatable otherwise...
If there weren't so much luck involved, I'd consider it a best turn-based strategy ever. This way, I've ended up banging my keyboyard against a wall every time I've played it for a longer time.