[meehalych]

Here are some examples of the way, how the Russian drivers communicate with each other, by means of the light signals and I wonder if there is anything similar in other countries (may be useful for those, who happen to travel to Russia by car):

One short flash by the distance light    - you may pass
two short flashes by the distance light - be careful there is a road police ahead
some long flashes from the rear car      - give me the way
emergency signal (not sure if it is called like this in English) flashes for 2 to 3 times - the driver thanks you for letting him pass by and etc
if there is a long truck ahead of your car on the road coming the same direction and its left winkers are flashing  -  do not outrun me, there is an obstacle ahead
If the truck's right winkers are flashing  - you may outrun me, the road is free

There are also some signals by hand, but those above are more often used on the roads.

[Dazzler]

Handy info Misha 

[asathorny]

Oh yeah, we have some signals by hand, well, actually it's more finger really......   

[eye30]

In the UK the only official signals given by lights are from the indicators.

Any signals given by flashing lights are not official and drivers should not act on them unless they are sure to do so will not cause an accident.

The most used unofficial signals are:
Flash of head light = you come on through.

Head light on constant = I'm coming through - although I would never use this as I bet the other driver would take it as - Flash of headlight - and could result in an accident if both cars try to proceed at the same time....