Who is Eglah?
Rashi, from Shochar Tov 59: It is Michal 1 , who was dear to him. Also in Shoftim (14:18, this refers to a wife) "Lulai Charashtem b'Eglasi."
Radak #1: It is a different wife. Michal is not mentioned here, for she did not have a son. Here the Torah lists David's sons, and with them, their mothers.
Radak #2: She was Sha'ul's wife. 2 David married her after Sha'ul died - "v'Es Neshei Adonecha b'Cheikecha" (12:8). Even though one may not marry a king's widow, a king may marry a king's widow (Sanhedrin 18a).
Radak (5:13): Sanhedrin 21a was forced to say so, for we learn that a king may marry only 18 wives from "v'Osifah Lecha ka'Henah vecha'Henah" (12:8; another six and another six.), so we must say that Michal was among the six listed.
Radak: Even though she was the wife of David's father-in-law (she was not his mother-in-law; Michal was from a different wife of Sha'ul), one may marry his father-in-law's wife. The Yerushalmi (Yevamos 2:4) forbids, but this is only mid'Rabanan due to Maris ha'Ayin. It says that David married Ritzpah, Sha'ul's concubine, for a wife. (According to the opinion that a concubine has Kidushin (refer to Shoftim 8:31:1:1,2 and the notes there), she is like a wife regarding Arayos. - PF)
Why is only Eglah called Eshes David?
Radak #1: This is Michal; she was his first wife.
Radak #2: (She was not Michal.) She was not from a great family, so she is not called by her family's name, rather, Eshes David.
Radak #3, according to R. Yehudah (Sanhedrin 18a): She was Sha'ul's widow. She could not be a wife to anyone else, only to David, for only a king may marry a king's widow.
Malbim: This is Michal; she was proper for David, for she was a king's daughter.
QUESTIONS ON RASHI
Rashi writes that "Eglah" is Michal. Below (6:23), it says that she did not have a child until the day she died!
