Ok, this has cropped up on the forums a fair few times. And there's a lot of advice spread over these forums.

I spent 9 months last year on fieldwork in Kyiv with only a Samsung NC10, and so have a fair bit of experience with what works and what does not, and some specific advice for certain games. So thought I'd put this together.

I'll keep it edited and updated as people suggest things. So please come forward with any suggestions, additions, corrections and game specific details.

1. Operating system
This will be one of the major impacts on what you can play. If you are thinking of buying a netbook and intend to game on it (or have one and are thinking of changing OS) then bear this in mind.

For Google chrome OS, you're limited pretty much entirely to their 'webstore' (which actually includes a lot of ports of iOS games, so no real bad thing)

For Linux, which will usually be a version of Ubuntu or maybe EeePCs own version, there are a range of games which run natively on Linux, and many which will run via DosBox for Dos compatible games and Wine for many windows games. See below for some more on wine

Win XP is probably the easiest netbook platform for gaming. DosBox runs fine, and it's compatible by and large with many older windows games.

Win 7 is just horrible for backwards compatability in many cases, but, you know, it's well fancy and pretty compared to XP...

2. Screen resolution
Probably the most important 'hardware' question. Beyond memory (you'll have enough) and integrated graphics card, the resolution of your screen will most detrmine the limits of what you can play.

My netbook has a resolution of 1024x600, which is fairly common. 800x600 is common too. Then there are some really odd ones...

The problem? Many games, even if they will run on your hardware, will not fit to 800x600. You'll have the bottom or top of the screen cut off.

Many netbooks allow for screen scrolling, where you can pan around a screen of a higher resolution by moving the mouse pointer to the edge of the screen. If not natively, this can be done using some of the software linked below.

This is annoying. At least I find it really annoying. Whilst mileage varying and so on, this is something to be very much aware of. Double check 800x600 is a possible resolution for any game (which will be the case for anything DOS usually)

IF you get a game that requires 1024x800 as a min resolution, there may be solutions.

Oddly enough, the best place for solutions to this (and to get 800x600 games using a full 1024x600) is the Widescreen Gaming Forum and the like. For whilst it's aimed at high resolutions, often the solutions will work for an arbitray given resolution. The games list is here:
The forums are worth searching too.
Try also:

These aren't guaranteed, though. Case in point: Galactic Civilisations II allows for arbitrary resolutions via any ini tweak. but anything below 800 tall makes the UI unusable.

Software which may help:
Netbook Resolution CustomiserAllows for down and upscaling. This will squeeze 1024x800 down to 800x600 if needs be. May crash some games though. IntelGMA required.
QuikRes Allows switching between resolutions from the tray. XP required.
Windows 7 downscales natively with a registry tweak:

3. Mouse
Get one, a cheap, small, wireless one. Even if you're not playing any FPS or similar games.
Unless, of course, you have some kind of alien coordination that makes a trackpad seem like a perfectly normal input for a computer. You wierdo.

4. Boost your hardware
There's a range of options here (most aimed at windows)

Overclocking tends to be Bad Idea (TM) on netbooks, as they lack and real ability to expel heat. But a modest 10-20% boost is within reason. No full details here as overclocking is always a fidlly business. So this is just some links to useful software and resources:

Anything IntelAtom:
SetFSB: Allows a range of tweaks of CPU parameters, useful for Overclocking.

Eeectl: Designed just for EeePCs this provides a huge amount of 'under the hood' control.

MSI Wind:
1.09 BIOS Allows overclocking options in the BIOS (inc GPU) for MSI Wind netbooks

A1ctl No overclcoking here, but some useful fan and resolution and memory functions.

For IntelGMA graphics (you may have something by Nvidia, but unlikely):
GMA Booster
Incredibly nifty thing that ups the clocks on your GMA integrated GPU without any fiddling with the voltages. Very much Worth It.

5. Flash gaming
I thought I'd add something on this seperate from the rest.
Flash is not limited just to browser games. Binding of Isaac, for example, is a flash game.
The problem with flash games is that they utilise the CPU exclusivley, and often poorly. The GPU utilisation by Flash only gets used for video decoding, not 2D rendering.

This often results in, unintuitvley, 2D games running worse than, say, a 3D game from 2002. Sometimes really, really poorly - it'll chug and stutter.

Other browser plugins are often much better.

For example, something that pushes flash's capabilities like The Last Stand may be unplayable, but full 3D Quake Live or a Unity game may be fine.

You may see something that looks perfect for a netbook, but check it's not a Flash game first.
Some Flash games run fine, others hardly at all. And it varies enormously depending on software and hardware configurations.

6. Behind the Games
Will probably be DosBox or Wine. Or even an emulator if that appeals. Or maybe some windows backwards compatability jiggery pokery.

Often games can be made to run much, much better with a bit of specific tweaking of whatever they are running on. Some of this may be game specific, but there are some various things to bear in mind.

apt-get if you can, else:

DLL overrides and graphics settings are likely to be the most common tweak:

AppDb, a database of guides on different apps and games is the most useful first-point-of-call for Wine. Lots of games are documented here:

Get it here
And get a [url=http://www.dosbox.com/wiki/DOSBoxFrontends]Frontendp/url]
A frontend allows for both a catalogue of games, for editing the conf file (which, yeah, can be done by hand I suppose...) and often comes with downloadable databases of game-specific configurations.
D-Box is very pretty, but without a full set of options for tweaking. (Link in above)

D-Fend (windows) of the multi-platform DBGL (links in above) are my favourites. D-Fend will even connect to a database of profiles and automatically add it to a recognised game. (However this is a bit hit and miss, you may have to tweak it yourself)

The best places for game-specific DOSBox info are at:
DOSBox games list
DOSBox Wiki: Games
Or search / ask at VOGONS fourms

Old windows games, compatability and hyperthreading problems
Will more likely run better on wine than Win 7...
There's not much to say here in general, as many fixes are games-specific.

But the usual Right click the .exe -> Properties -> Compatability -> 'Run as admin', 'Run in compatability mode for' and 'Disable visual themes'
Is the best place to start.

One thing that does appear often are games that fall apart in a multi-thread environment.
This is the Task Manager -> Right Click -> 'Set affinity' solution you're probably familiar with.
If this is a problem for the game you want to run, this can be done permanently in XP, Vista and Win 7 by following:
OR this can be done permanently by editing the .exe file itself by downloading and using the command-line program imagecfg:
(Backup the original, obv.)

Other than DOS or windows...
Just a list for reference:
ScummVMFor old adventure games
The Emulator Zone

7. Games and where to get them
First off, aim high! Anything more than 4 years old will probably be ok... As long as resolution is not a problem...
Whilst a lot of the above relates to older games, but you'd be surprised what will run on a netbook.
My NC10 had no problem with KotOR one and two (minus a bit of slow down in really busy areas)
Some source games (HL2, Portal) may also be fine. Gothic 11, Morrowind (with extra low settings) and hitman 2 are fine.
So no -need- to go for the ancient games.

For the NC10 there's a list here:
Other netbooks may have their own lists.

That being said, half the point of gaming on a netbook is an excuse to play classics again (or for the first times)

I'll get around to a list of suggestions and game-specific tweaks later. For now, the obvious starting place is, of course, gog.com...