RPi Screens

Supported Standards
The following display outputs are supported on the board:
 * Composite video (NTSC and PAL) via an RCA plug (the yellow socket on your TV) or SCART socket.
 * HDMI 1.3a standard output.
 * Display Serial Interface (DSI) - via unpopulated 15-way flat flex connector.

These interfaces allow the use of a broad range of displays, including:
 * Televisions (HDMI or composite).
 * Computer monitors (HDMI/DVI-D cable) (VGA monitors would require either a composite or HDMI to VGA or component scan converter box).
 * DLP/Laser Pico Projectors (HDMI/DVI-D cable).
 * RAW LCD panels (DSI + interfacing hardware)
 * Composite and HDMI outputs can not be used at the same time (one or the other.)

HDMI
Directly from the board.

DVI
With an inexpensive passive HDMI to DVI-D cable or convertor.

Composite video
Composite video (NTSC and PAL) via an RCA plug, directly from the board.

VGA/RGB analog
The Broadcom BCM2835 only provides HDMI output and composite output. RGB and other signals needed by RGB, S-VIDEO or VGA connectors are however not provided.

A couple of options for VGA or component RGB outputs, bridging from either HDMI or the MIPI DSI interface:

Firstly, the following *might* work. Beagleboard people have reported various levels of success (mainly "issues"):

http://www.hdfury.com/

Something similar:

http://www.fit-pc.com/web/fit-pc2/accessories/fit-vga/

...it may need to be modified as described here:

http://blog.galemin.com/2011/03/dvi-d-to-vga-converter-for-beagleboard-xm/

Alternatively, it may be possible to design an expansion board that plugs into the LCD headers on the R.Pi. Here is something similar for Beagleboard:

http://boardzoo.com/product_info12.php

Some more converters that should work:
 * $24.00 ASUS - Video adapter
 * $27.77 HDMI Male to VGA RGB Female Video Converter
 * $39.99 HDCP HDMI to VGA + SPDIF RGB Component Ypbpr Converter
 * $48.00 HP HDMI to VGA Display Adapter

Potential RGB interfacing chips:

They emit Hsync, Vsync and RGB w/ about 8 bits each (sometimes, it's called "MIPI DPI")
 * Toshiba TC358762XBG deserializer bridge chip
 * Analog Devices HDMI Receivers
 * Renesas µPD60800 Intelligent Buffer IC
 * Renesas µPD60801 Intelligent Buffer IC

The SOC (system on a chip) does not support any kind of analog component video, including VGA, since the SOC is designed for mobile phone use where this would not be a requirement. Additional componenty would be needed to generate RGB signals. Additional componentry would push the price beyond the $25 target and therefore won't happen.

interfacing to Raw LCD panels
Nokia N900 has 800x480 DCS LCD (afaik its like DSI, but has build in framebuffer). Replacement screen is about ~$40 (~$50 with 4pin resistive touchscreen).

Nokia N8 has AMOLED 360x640 pixels DSI LCD. Replacement screen is about ~$35, another ~$25 for Multitouch Synaptics T1021A touchscreen (I2C + interrupt IO, no docs/drivers).

Potential LVDS interfacing chips :

TC358764/5 Display Bridge (MIPI® DSI to LVDS)

DVI receiver TFP401A, TFP403, or TFP501 + LVDS transmitter SN75LVDS83B or SN65LVDS93A (Mentioned earlier fit-VGA is build around TFP401A, probably many more "active" DVI2VGA cables are build the same way)

I2C/SPI ADC can be used to interface 4 pin resistive Touch Screens, For example STMPE812A

16x2 LCD Displays
The old-style 2-line, 16-character LCD displays can be commonly controlled over UART, thus providing a cheap way to display values for sensors etc.