Peter Goranov wrote:(i don't know why some posters here regard katanas and Japanese swordsmanship with disdain and label it as commercial though)
Not Japanese swords and swordsmanship--I think what most people rail about here is just the
representation of Japanese swords and swordsmanship in popular culture, which does justice to neither European swordsmanship nor
real Japanese swordsmanship.
I know that when it came to steel armour, western warriors had a choice of mail (chainmai), scale or plate.
What period are you talking about? If it's the Middle Ages proper, I don't think scale (in the meaning of small metal pieces attached to a backing with each scale overlapped by the one above it) was a widespread method of protection among European warriors. However, lamellar (where the individual small plates are attached to each other rather than to a backing, usually with each scale overlapped by the one
below it) saw fairly widespread use among Eastern European warriors, and brigandine (small metal plates riveted to an outer fabric/leather covering) was quite popular from the 14th century onwards.
Of course, this is only valid for the Middle Ages, and other periods definitely used different standards.
It is a widespread "fact" (?) that plate is superior to the other two.
I'd echo Jeremiah here. Plate might give the most solid protection, but it wasn't the "best" or the most "superior" for any and all conditions. Even after plate armor became widespread, mail was still frequently used to cover the gaps in plate protection. Not to mention that the flexibility of mail made it handier for certain purposes, such as concealed protection beneath a coat or doublet.
My question is what was each type of armour created for, which weapons it countered best and how available was it to anyone but nobility.
Get a good book.

The answer ot the question is too long and vast to be contained within a discussion forum like this. The [url="http://www.thearma.org/reading.htm"]Research & Reading[/url] section of this very site contains many good bibliographic recommendations for the study of European arms and armor.
And of course how easy it was for one to move in it when not riding a war horse.
Generally speaking, very easy--at least when we're talking about field armors of the kind used in campaign and on the battlefield. In the later Middle Ages, there was also a different, more specialized kind of jousting/torunament armor that gave more protection for less mobility, but these were worn only for tournaments, not the battlefield.
My second question is about maces, especially two-handed maces and war hammers. Were they designed solely for use by mounted warriors?
No. Medieval illustrations also show them being used on foot,
especially the two-handed variety.
If not, than how does one go about carrying it around? Strapped to his back maybe?
In two words: "baggage train." Knights and men-at-arms at the time usually traveled with a considerable number of pages, servants, and attendants. and these people took care of giving the men-at-arms the right weapon for the right situation.
Furthermore how does one parry the enemy's blows using such a weapon? Having one end much heavier than the other would make it hard to use when deflecting blows from a lighter weapon (a cutting sword, for example a broadsword).
Armor and/or shields. A little time spent browsing medieval illustrations will show you that, medieval European maces, warhammers, and poleaxes were mostly wielded by men in more-or-less complete armor.
I know that these weapons perform well against plate armour, but did it matter where the blow landed or not? (excluding bashes to the head of course

)
It did. Most examples of genuine historical plate armor did not exhibit a uniform thickness throughout its surface, and hitting one of the thinner pieces or portions would give a better chance of wounding the enemy. Another factor that has to be taken into consideration is padding and suspension--when the men could aim their blows at all, they were naturally trained to aim at the portions where the padding or suspension systems would be least effective for absorbing the blow.
My third and final question is on two handed swords (greatswords?). What were they designed for - a counter to cavalry and formations of pikemen? How effective were they when used in actual war, not duels or jousts and such?
Well, it depends on
which two-handed swords. Perhaps you could start by looking at articles that attempt to define the characteristics of various sword forms, such as
http://www.thearma.org/terms4.htm (ARMA's own) and
http://www.myarmoury.com/feature_euroedge.html (myArmoury's).