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.

RGB analog/VGA
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 "Video Converter adapter" from china
 * $24.00 ASUS - Video adapter requires additional miniHDMI to standard HDMI adapter
 * $32.50 HDMI to VGA Video Converter
 * $39.99 HDCP HDMI to VGA + SPDIF RGB Component Ypbpr Converter
 * $48.00 HP HDMI to VGA Display Adapter

Potential interfacing chips:
 * TFP9431 DVI RECEIVER WITH TRIPLE DAC (hard to find, obsolete?)

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 components would be needed to generate RGB signals. Additional components would push the price beyond the $25 target and therefore won't happen.

RGB digital
A HDMI/DIV to parallel 3.3v interface PCB, also meant for the Beagleboard:

http://www.harbaum.org/till/dvi2par/index.shtml

Potential 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

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).

The schematics for apples iPhone 3gs and 4g suggest they speak DSI, thus they can probably be connected directly. The older iPhones use a "Mobile Pixel Link" connection from National Semiconductor. The 3GS panel (480×320) goes as low as US $14.88, while the 4G one (960×640, possibly the LG LH350WS1-SD01, with specifications) can be had for US $17.99 or as low as US $14.28. The connectors used might be an issue, but this connector might fit. Additional circuitry might be necessary to provide the display with required 1.8V and 5.7V for operation, and an even higher voltage for the backlight.

Other panels with MIPI DSI: www.panelook.com

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. Texas Instruments has a solution for 4 or 8 wire touchscreens using their rather cheap MSP4309.

An additional binary blob might be required for the DSI port to function correctly (or function at all). When or if such a blob will be made available is unknown.

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.