There are several screens that are commonly used. Here's a (incomplete) list. This is not supposed to be in-depth, just a nice overview.

Key:
C=accepts composite video
S=accepts S-video
RGB=accepts RGB video

1. PSOne Screen (C/RGB) (7-9V)(PAL or NTSC versions)(model no SCPH-131)
The creme de la creme of screens. At 5 inches, it's about the right size for most homemade portables. The board is larger than the screen but can be trimmed. It takes composite and RGB and requires about 7-9V. It is a very well documented screen; plenty of info on it. Consumes about 400mA with a LED mod, 700ma without. Unfortunately, it has become somewhat expensive to get. It can be purchased slightly cheaper as a Zenith PS2 screen. The Zenith is exactly the same screen but has a different input board and speakers. There are other recases out there as well. Probably the best screen for portabilizing.
PinoutLED Mod

(picture shamelessly ripped from eBay)

2. Pustom 2.5"/3" Screens (C)(12V, has been run down to 5V)(NTSC/PAL)
These screens are cheap and decent. Not great, but decent. Small, good for small portables. It only accepts composite unfortunately. It's designed for 12V but I've heard it being run down to 5V. Has built in mono crap sound I think. Cheap and readily availible from DealExtreme or eBay.


3. AEIComp 2.5"/3.5" Screens (C)(3.3V-12V)(NTSC/PAL)
These screens are expensive but good. You have to do a considerable amount of hacking- they have a large unnecessary input board and no audio amp. Has an LED backlight already-very power efficient. I'm not going to go through this in detail; instead I will refer you to Rekarp's excellent guide.

4. Hipgear/Intec Screenpad (C/???)(9V+, 5V+ with regulator removed)(NTSC, possibly PAL availible)
I don't know a lot about this screen, other than it is 1.8" and is attached to a PS2 controller. Perhaps someone can fill me in. As far as I know the Intec and Hipgear versions are the same. Power consumption varies with voltage. I think a GC version is availible as well.
MORE INFO!


Other Screens you can use:

1. Any AEIComp screen that accepts composite or RGB.
2. A computer monitor or small TV that accepts S-video, composite, or sometimes component or VGA.
3. A pocket TV.
4. Any sort of game screen, but the Intec and most others are total crap.

Screens that you CAN'T use:

1. Game boy, game gear, DS, or PSP screens.
2. iPod or MP3 screens.
3. Most portable DVD players.
4. Cellphones.
5. Pretty much anything that doesn't take a normal signal.
Screens that can sorta be used:
1. Portable DVD players, certain types. You have to keep the main board.
2. GBA. You need an input cart and the whole friggin' gameboy.
3. Nomad screen. Accepts RGB, but expensive and crap.
4. Laptop LCD. If you have a certain type and about $200 to blow on an encoder, that is.

More is coming soon, more links, more screens, maybe more information. Stay tuned!

EDIT: Screens that can and can't and can sorta be used.
EDIT 2: Moar info!

Another important note about AEI screens is that you will need to build your own audio amp. Not hard at all, but it is an extra step.

Added.

Where have you heard about the Pustom screens being able to be run off of 5v?

I ordered one a couple of days ago so I'll let you all know what the real deal is when it comes in.

I forget, but I think someone said there's a regulator inside that can be bypassed. Of course the backlight won't work at 5V, so you need to LED mod.

I heard from Palmertech.

PAL PSone screens can take S-Video.
viewtopic.php?f=8&t=20158

I know, but I didn't mention it because it takes quite a bit of modding and only applies to PAL screens. Maybe I'll add it in the next update.

Read an article the other day saying you can also get S-video from NTSC screens, using the multiport connections. Not tried it though.

Oh great, now I have to add another screen

I'm wondering about other names for the psone screen. I know Zenith makes some. Is Intec another? examples: http://www.amazon.com/5-6-INCH-COLOR-GA ... 21&sr=8-12
http://www.amazon.com/Intec-Black-Game- ... 21&sr=8-16

I also remember seeing a manufacturer that started with a "J" in someone's tutorial somewhere...
BTW- what is the actual diagonal measurement of the screen? 5", 5.4", 5.5", 5.6"? I seem to see slight variations the more places I look.

Noooo Intec is not. Those screens are horrible.

Can somebody give a link to those Pustom screens on dealextreme? I can't seem to find them.