A short but very tricky filler "aber"

The so called "fillers" are words, that do not have any particular meaning. They "just" fill the speech. Often they can not be translated into the other language and that's why cause difficulties for the people, learning this language.
Of course, there are "fillers" in German too: "doch", "mal", "bloß", "halt", "etwa" etc. We will discuss all of them by DeutschMe. However I would like to start with "aber". "Aber?"-, you ask surprised. But "aber" is a not a "filler", it is a conjunction meaning "but"! Yes, that's true, but that's what is so tricky about "aber"- it is very widely used not just as a coordinating conjunction in cases like "Ich will spazieren gehen, aber es regnet" (I want to go for a walk, but it's raining) or "schön aber zu teuer" (nice but to expensive), but also as a particular or a "filler".

In the most cases "aber" reinforces the statement: 

Aber ja	Yes indeed; yes of course!
Aber nein	goodness, no
Das ist aber schade	What a shame!
Das war aber knapp	That was a close call (shave)
Aber selbstverständlich	but of course
Aber natürlich
Aber klar doch!
Aber gerne	with great pleasure!

aber (wirklich):
das ist aber schön!	that really is wonderful!
das ist aber nicht gerade nett von dir!	that's not really very nice of you, is it!
aber (empört):
aber	oh
aber Hannelore, reiß dich doch endlich zusammen!	[oh] Hannelore, pull yourself together!
aber hallo!	Excuse me! (emph)

It is even used as a noun

Die Sache hat ein Aber	There is just one snag
Kein Aber	no buts

http://german.about.com/library/weekly/aa010806a.htm