In my Church's tradition, there are many congregational responses. In addition to "Lord have mercy" that is common to all of us, as mentioned by HTacianas in post #10, we have responses for the Gospel reading, the Praxis (this is what we call the reading of the Acts of the Apostles), and so on. (I've been told by EO people on this website that the Coptic Orthodox Church has more readings during its liturgies than other churches do.) These have their own variations according to season. Like here is the "annual" (Arabic: سنوى sinawi) Gospel response, to be said on days without specific feasts or fasts:
The shorter, simpler responses you're probably thinking of after seeing "Congregational response" are a lot of "Amen"s, "it is meet and right", "we have them with the Lord" (referring to our hearts), "And with your spirit" (to the priest, in response to his "Peace be with you" -- this is another thing that seems to be common across all historic liturgical churches), etc. Those are all over the liturgy.
There is also the very common antiphonal (I think that's the word...) response types, wherein the priest or deacon will begin the proclamation, say before reading the Gospel, and the laity will finish it, as below. The deacon (now priest!) intones "May his blessings" (referring to those the Gospel writer) and the people respond "...be with us all, amen." I always kinda like that...probably because it's the closest I'll ever get to being a deacon.
(Maybe it should be stated here that the Orthodox liturgies
require this participation -- i.e., there is no notion of a mass said absent the people, as there is in the Catholic Church with its
'private' masses. There are things you can do if you can't get to liturgy, but you can't have a liturgy by yourself.)