The Minhag is to fill up an additional cup towards the end of the seder known as Kos Shel Eliyahu. I was just wondering what the various minhagim are as to what to do with it after the seder is over.
My father always used it the next day for Kiddush, and that is what I do.
The Chabad minhag is to pour it back into the bottle after לשנה הבאה בירושלים. While this is being done, everyone at the table sings א-לי אתה ואודך to the tune composed by R' Shneur Zalman of Liadi.
IIRC my father pours it back into the bottle (without first depugmifying it).
My great-grandfather's practice was to pour wine from the Kos shel Eliyahu into each person's fourth cup. (My grandmother used a spoon.)
I don't have a source or even a reason for the practice, but when I asked around I was told that it was an affirmation of early Religious Zionism, since Eliyahu's cup represents the fifth expression of redemption, "v'heveti," G-d's bringing us to the land of Israel. To show that this redemption has started again, everyone drinks from the wine.
Nechama Leibowitz' Haggada mentions that after establishment of the State of Israel Rabbi Kasher tried to convince the Rabbinate to add the fifth cup at the Seder.
הגדה של פסח עם פסקי הלכה ומנהגים ממרן עמוד ההוראה הגאון רבי שלמה זלמן אויערבאך זצוק״ל, by ישראל יוסף ברונשטיין, published 5764, page 218, cites from ויגד משה 30:5, in the name of קונטרס מנהגי החת״ם סופר, that since the cup was used for one מצוה it should be used for another, so some leave it covered until the morning and make קִדוש on it then. It further cites ויגד משה 30:5 as saying that some pour from the cup into the various cups of the people at the סדר after "שפוך חמתך", but adds that that would only be for those who fill the fourth cup after saying "שפוך חמתך", which, it says, is not the prevalent custom.
Eliezer Brodt has a post on the Seforim blog where he analyzes the development of this custom in great depth: The Cup for the Visitor: What lies behind the Kos Shel Eliyahu?
"There are definitely early sources that talk about a fifth cup of wine at the seder. According to some, this fifth cup at some point started being called Kos Shel Eliyahu. Starting from the late 1400s we find that people would pour a special kos, and call it Kos Shel Eliyahu. I have shown that there are early sources for opening the door at Shefoch Chamascha that give various reasons. I also showed that there are many drawings by Shefoch Chamascha of a man on a donkey and Eliyahu found in the early manuscripts and printed illustrated Haggadahs. This would logically lead us to conclude that there was a belief that he did indeed come to visit when the door is opened."
In terms of what to do with the cup after the seder, some of the sources he brings include: R. Hayyim Benveniste (1603-1673), who writes that Ashkenazim empty their cups at the end into a fifth cup that they call Elijah's Cup, and he adopted this custom and drank it; and Rabbi Moshe Hagiz (1671-1751) who writes that after the seder the head of the household sleeps next to the full cup.