[puyofan99]

I do not normally post on forums, but I wanted to. As always explained as is (sorry if saying "title explains everything" is a part of the rules), so Title explains everything. 

[Jorpho]

Well, no, it doesn't, actually.  What do you consider "small"? Is this for Windows, DOS, or what?  What sort of system requirements are you thinking of? Does this need to run from a USB thumb drive? By "free", do you mean "legitimately released as freeware", or do you count old stuff that is no longer sold?

Asking detailed questions is very helpful on forums. You may enjoy How to Ask Questions the Smart Way.

[FAST6191]

"Does this need to run from a USB thumb drive?"
Secondarily to that if you are trying this on some janky cheap laptop or something that came with no storage but offered "cloud storage" to bump up the space then you might wish to look up the term "portable". That way you can happily stick things on a USB drive, SD card or similar.

https://portableapps.com/apps/games is a more dedicated thing for portable versions of open source games, https://pendriveapps.com/software/portable-games/ has some more. I am not sure where to go right now to find people that are doing their own portable versions of things not originally designed for it, and I am not sure you even need to go full bore portable for your needs (you presumably do have the registry and option to keep save games wherever).

https://www.traynier.com/software/steammover is also something to consider in this. It allows you to change the location of games on Steam and thus boot them from another drive.

Depending upon what you are doing there are also plenty of browser games, and depending upon where you are then remote/streaming gaming is also a thing that is at least possible (though most implementations leave a lot to be desired).

[xZabuzax]

Go for Cave Story (Doukutsu Monogatari), it's the best freeware game IMHO, it's small, free, kind of long and the characters, OST and gameplay are phenomenal.

[Jorpho]

I thought about Cave Story, but I'm not sure what the "smallest" way to download it is anymore.  I know there's a whole bunch of different re-implementations of the engine and it looks like the most up to date "deluxe" package is 75 megabytes.

I also thought of E. Hashimoto's trilogy – Akuji the Demon, Guardian of Paradise, and Hydra Castle Labyrinth – but I'm not sure those have good replay value. (They're certainly fun enough while they last.)

[Mauno]

La-Mulana is awesome.
I was going to suggest Legerdemain but didn't realize how big it was, it's over 50MB for the version that doesn't require you to have anything else installed.
Holdover (3rd menu option is settings where you can change language to English)
Spelunky
Cave Story
Mindustry it's almost 100MB though

[aqualung]

If you like Metroidvanias, you can give Timespinner a try. It looks and plays exactly as the gba and ds castlevania games and it has a fairly nice story (for these kind of games, at least) involving time travel. I enjoyed it a lot.

Edit: sorry, I somehow overlooked that you asked it to be a free game as well. This game isn't free, but I don't think it is very expensive either.

[Hemlock]

If you want to play a FPS game that is small on disk space, try .kkrieger. It only takes 96 KB.

[Jorpho]

I just remembered ROM Check Fail. Lovely little pick-up-and-play game, and not very big.  Apparently playable in a browser now, too.
http://farbs.org/

Natomic's Super Stun-N-Run also quietly comes to mind, but it seems their website is down and I'm not too sure where you would find it anymore. They also made The Spirit Engine and The Spirit Engine 2, but I don't think those are especially small (but they are free and totally worth playing).

[Chronosplit]

I don't know how you define small, but Hedgewars should also fit the bill.

Most free roguelikes you find are also pretty small, as there isn't much data in graphics.  The only exception to this I can think of is Tales of Maj'Eyal, but it's also not 100% free.

[Blomman]

I recently played Starhips Unlimited again. It's free and 17 MB download. It's a space strategy game that's lighter than those usually are and focuses more on ship-to-ship combat. Available at apezone.com

[SleepyFist]

Here's a copy of my free indie games list, it's by no means comprehensive, but I assure you they're all good, enjoy

Iji
http://www.remar.se/daniel/iji.php/

Hero Core
http://www.remar.se/daniel/herocore.php

Cave Story
https://www.cavestory.org/download/cave-story.php

La-Mulana
https://archive.org/details/La-Mulana

Yume Nikki (Non-Steam version)
https://www.dropbox.com/s/m0xbpqveq5rucik/yumenikki0.10eng.exe?dl=0

Yume 2kki
https://yume2kki.fandom.com/wiki/Downloads

.flow
https://yumenikkifg.fandom.com/wiki/.flow

1bitheart
https://vgperson.com/games/1bitheart.htm

1beatheart
https://vgperson.com/games/1beatheart.htm

Runman:  Race around the world
http://wherecouldtom.be/games/runman/download.php

Off
https://off.fandom.com/wiki/Downloads

Space Funeral
https://gamejolt.com/games/space-funeral/3492

Kiwi 64
https://gamejolt.com/games/kiwi-64/33914

Presentable Liberty
https://gamejolt.com/games/menagerie2/42997

Strange Town Series
https://gamejolt.com/@garmentdistrict/games

Action Fist
https://www.dropbox.com/s/2qlxegpscze1ecz/Action-Fist_Win_EN.zip?dl=0

Arcane Kids Turbo Funtime Activities Pack
https://mega.nz/file/HAEHha7S#Gj8ZDIml06lqvC_Yxmci0l5IE8lhOFgVIfLFk8Pwzow

[mkwong98]

Eternal Daughter is a very good Metroidvania game.

[Djungelurban]

An Untitled Story is thoroughly underrated...

[foiopaulo]

Final Vision is a platformer/metroidvania with Final Fantasy inspired characters.

[SleepyFist]

An Untitled Story is thoroughly underrated...

I completely forgot about that game, it's a good one.

Gotta recommend Modarchive Story and Knytt Stories too.

[Recca]

If you're looking to play some old school retro DOS games, I would recommend Highway Hunter and One Must Fall: 2097. They're both now listed as abandonware can easily be found on the web and played using DOSBox on most newer computers. If you're interested in some freeware games, try Cyberdogs and C-Dogs which were both made by Ronny Wester back in the late 90s and early 2000s.

There's also a large list of various homebrew games available here that you can try as well. Anguna for the GBA and DS is a good one.

[gamingcat02261991]

Cave Story's the only game I can think of. Oh, and Yume Nikki. And, to tell you the truth, with my current internet connection, I don't think I'll be downloading games above 1GB in size due to the connection throttling once the 1GB limit is reached (no thanks to Spectrum).