Internal Flash

The internal flash of the Canon S20 is located to the left of the lens and produces fairly good results up close but has little power to light anything distant. This is confirmed by the manual which defines the working range as:

Wide Angle: 17cm - 3.3m (6.7" - 10.8ft)
Telephoto: 17cm - 2.3m (6.7" - 7.5ft)

Skin tone flash test, sorry Canon but you've got something seriously wrong with the colour balance here (see note below) Macro flash test, the flash is very well measured and never washes out its subject this shot is a good example of that.
Coped well with a difficult subject, no wash-out and well lit. About 3m from this plain white wall at full wide angle (supposedly within the flash range limits) but as you can see there's drop-off at the edges.

These simple flash tests concluded my original analysis that the Flash on the S20 is (a) underpowered and small and (b) had an obvious yellow colour shift which had the nasty side-effect of making people in flash shots look very yellow. That said it's difficult to produce such a compact camera with a good flash, I could have forgiven it's lack of power if the colour balance had been better.

Focus Assist Light

The focus assist lamp is the window on the left in this image. This is a shot taken with my trusty 950 of the focus light projected onto a wall, as you can see it's a bright circular area with two horizontal bars. (not as bright or yellow to the human eye).

The S20 features something not found on other digicams (other than the Canon Pro 70), that is a focus assist light (seems to be combination of visible and infrared light), the actual emitter is a small lamp to the right (from the back) of the viewfinder window. It emits a light in low-light situations which assists the internal auto focus in focusing in the dark. The S20 can focus where so many other digicams would fail (Nikon, are you listening?).

Preset IMAGE modes

The S20 features a special "IMAGE" mode which puts the camera into preset modes for shooting particular types of scene or special effects. This is a neat arrangement and makes it easy for the user (beginner or advanced) to capture the effect they require without having to make too many settings. Each mode is indicated by an icon on a selector bar as shown below.

Landscape mode locks the cameras focus system to infinity and cancels the flash.
Fast shutter takes shots with a larger aperture (smaller F-number, smaller depth of field) to try to keep shutter speeds as high as possible.
Slow shutter takes long exposure shots by using a small aperture (larger F-number, greater depth of field) and allows the camera to take exposures up to 2 seconds in length.
Night Scene (slow sync) takes long exposure shots with an initial fire of the flash (known as slow-sync on other cameras). Useful for taking shots of people at night against a dark background.

Black & White mode on the S20 causes the CCD processing algorithm to discard colour information to produce a grayscale output. Oddly the grayscale output from the S20 has a slight purple cast (R+2, B+3) which is probably a throughback to the fact that the S20 uses the CYGM colour filter.