What's the best freeware game you've played? (It has to be completely free with no paid dlc)

[Alexinator]

For me it was Cave Story. It was a large metroidvania game with multiple endings, loads of collectibles and a beautiful soundtrack.

[Aaron]

For me it's definitely Tyrian 2000, love the soundtrack and the gameplay is really fun. Although it's the first shoot-em-up I've ever played, I found it really good and I highly recommend it to anyone that likes retro games and shoot-em-ups. I haven't played it in quite a while though.

[Raymanarenaps2]

Super Tux, originally a Linux only game, you can now play this game everywhere pretty much. Since it is open source, also freeware, anyone can freely play and even take the source code and make there own version of it using the same engine and even same sprites. It's good if you want to play free platforming games and you want to avoid having to worry about companies like Nintendo interfering with your emulation experience. That's why I recommend Super Tux. That and supertuxkart.

[Steo]

Solitaire and Pinball on Windows XP ftw

[Ambidextroid]

I think the freeware game I've gotten the most mileage out of has probably been Osu. There are a practically limitless number of songs you can play and 4 game modes which are actually just 4 totally different rhythm games in one.
I suppose I'd also say TF2. I'm not sure it counts since you can buy items, though I'm pretty sure you can unlock everything (except cosmetics) without paying.
World of Tanks is also really fun, and while it does have P2W and premium content you still pretty much get the full experience without paying anything.