I've been wondering what is Meta-key exactly to activate numerous desktop effects? Point is I use Left Windows key to switch my keyboard layouts and somehow I guess it is the key to activate desktop effects. Am I right in this assumption? If yes how can I assign any other key, say Right Alt, to be this Meta-key? If no what is this Meta-key exactly?

In general the Meta key is the "Windows" key. You can test the key with "xev" running in a Konsole. But there the Meta key is called "Super". You can change the keys either in the keyboard settings control module (I think under Regional&Language) or you have to use xmodmap.

It does depend on the program; for some ex-Unix programs, it may simply be Alt. In these cases the Windows key may play a different function.

How does one remap Meta-key to a key of choice?

You will need to use Xmodmap to do this. Please see http://www.macosxhints.com/article.php? ... 5231058862 for an example on how this is done ( note that some steps do not apply )