We mention 4 main types of DIY multitouch technologies.

FTIR Method
FTIR stands for Frustrated Total Internal Reflection.
This is currently the most popular method, possibly due to the wide internet distribution of Jeff Han’s videos.

IR light is shined into the side of an acrylic panel.  This light is trapped inside by internal reflection.  When a finger touches the acrylic surface this light is frustrated causing the light to scatter downwards where it is picked up by a IR camera.

Diffused Illumination
IR light is shined at the screen from either below or above the surface.
Citywall, Touchlight, Reactivision, and MS Surface use these methods.

IR light is mounted underneath a glass/acrylic sheet with a diffuser.  When an object touches the surface it reflects more light than the diffuser or objects in the background, the extra light is sensed by a camera.

Depending on the diffuser this method can also detect hover, and objects placed on the surface.

Gap Method
Unsure about the popular name of this technique, so we shall call it “Gap” for now.

A wide baffle is placed around the edges and above the glass/acrylic surface.  This baffle leaves a little gap (0.5mm) between itself and the acrylic surface.  IR light is shone though this tiny gap creating a small very narrow beam of light over the acrylic surface.  This beam is broken when the user touches the surface, and the IR light is detected by a camera mounted underneath. 

LEDs should be mounted on all 4 sides.

A sensor based technique
AMLCD Optical Touchscreen

Capacitance Testing
This is currently how the iPhone does it sensing, see Fingerworks. This method is very costly due to the complex electronical grid needed. See: http://projects.dimension-x.net/archives/111