xserver-xorg-video-nouveau, Linux Mint's and Ubuntu's default graphics driver
1:1.0.10-1ubuntu2

What runs?

Kega Fusion
QuakeSpasm 64 bit
ZDoom (Doom/Doom II/Strife)
Neo Geo Linux ports (64 bit tested); Art of Fighting 2, Blazing Star, Fatal Fury Special, Garou, Ironclad, King of Fighters 2002, The Last Blade, Metal Slug 1/2/X/3, Pulstar, Real Bout Fatal Fury 2, Samurai Showdown 2/V.

Armed Seven (32 bit is all that's available, but runs great)
Defender's Quest (nice-n-smooth)
Duke Nukem 3D Atomic Edition
Fist Puncher
Gigantic Army (32 bit, no 64 bit as of this test)
Retro City Rampage DX (like butter)
Towerfall: Ascension (silky smooth)

What doesn't run?

Droid Assault
ioQuake3 (64 bit tested)
Race the Sun
Revenge of the Titans
Strife Veteran Edition
Titan Attacks
Ultratron
Unreal Tournament via 451 Rev4 Linux patch

What runs like crap?

Torchlight II (runs but stutters pretty bad)
Gateways (runs very stuttery)

Probably not, but who knows?

This is absolutely not correct. First OP should state which GPU model he is using.
With nouveau it is like this - its a russian roulette. Sometimes, GPU is completely supported, sometimes the support state is garbage. For example g80/92-based are fully supported.

This is only because, unlike Intel and AMD, Nvidia does not back up nouveau and only supports its own proprietary driver. Its Nvidia fault and if you support Nvidia, you support this type of driver. For example, Linux kernel dev lists do not accept bugreports if nvidia driver is running.

You should change topic header to "Nouveau default Linux driver", because:
- I am using (and have been using) radeon driver on high-end cards, which runs everything
- the "xserver-xorg-video*" is only very small part of nouveau driver (Xorg DDX), technically its just a thin layer that allows to bypass Xorg completely. The driver is actually called "nouveau" and its two-component, with libdrm-nouveau2 in the userspace and nouveau in kernel.

If someone wants to understand better how Xorg driver stack was radically changed with introduction of DRI, here is the good page to see the history of changes around Xorg and DRI, and a good video of how it evolved.

All proprietary drivers rely on DRI likewise, they just implement own gl library and provide own kernel driver (via module). The xorg part of nvidia driver is likewise useless and exists to bypass its protocol.

Edit: drm page explains even better than dri page.

This paragraph is even better, shorter overview.

I use a propietary Nvidia driver: 367.57 with a GT 720 (900MHz).

I switched to xserver-xorg-video-nouveau as a test, just to see what it could run. Then, I went back into the Software Manager and switched back to Nvidia 367.57.

I do intend to swap out my GT 720 for an AMD Radeon HD 6450. Mainly because I don't play high-end games, I want low power consumption (18W), and I like AMD's DRM-free (no Software Manager nor Repositories required) drivers.

To me, DRM-free goes hand-in-hand with open-source. If it's gotta go through Repositories, then, it's DRMed. I'll take freedom over power.

I'm aware of Nvidia's not-so-Linux-friendly stance. Linus Tovalds has publicly gone in on them a few times over it. I'll be switching to an AMD GPU in the near future. I like that AMD's Linux drivers are DRM-free; not requiring Repositories for access or installation. Nvidia's drivers, on the other hand, won't install without going through the Software Manager - online.

Since my gaming tastes are rather retro, I don't need a high-end card: a 512MB low profile card will do me just fine. I'm eyeballing the Radeon HD 6450 (1GB or 2GB), and admire its low power consumption (18W). I already have the .deb driver installers for both 64 bit and 32 bit. DRM-free = freedom. I'll take freedom over power.

I wouldn't use ethics to pick companies, really. Companies are here to make money, they are greedy for control.
As such when you are paying, you are getting what you are getting. See its pros and cons and use what you think suits you better.

The best example is recent AMD change - while they have basically written modern Linux graphics stack for ALL gpus (Gallium) and invested a lot into what Mesa today is, they do not particularly shine in CPU area. They closed up specs for Coreboot in recent cpus, because they wanted to implement software-only version of TPM in firmware(!) (called "fTPM"), they cut AGESA code etc.

Also AMD GPUs are not entirely floss, the card require firmware loaded (golden bits) to unlock video decode and operate 3d. You will not be getting this if you don't install firmware. They claim this is how hardware works in contrast to Intels. Intel gpus do not require any in-kernel firmware at all, but why? Because its loaded automatically - Intels startcode has always been black box. And instead of coreboot, Intel created "UEFI".

Look up what you get for money, what you will be dealing with and if this is ok for you. No future promises.
I am on AMD because I like what they do and because I can be completely sure my card will be 100% supported for many next decades. Any card from hd3xxx and up will work flawless, but if your goal is video decode, you better get the card with best decoding engine.

Basically this: https://en.wikipedia.org/wiki/Unified_Video_Decoder#UVD_6
You want to pick a card with latest GCN generation.