In Psalm 81:6, why is Joseph's name spelled יהוסף? Seems to be the only place in the TaNaK that I currently know of that his name is spelled in this fashion.
Ibn Ezra says it's a more glorious version of Joseph's name, used here because "Joseph" refers to the Jewish people.
Sefer HaYoshor to Parshas Mikkeitz relates the following story:
"It was on that night that G-d sent one angel from His angels and attendants before Him and it came to the land of Egypt to Yoseph. The angel of G-d stood by Yoseph and behold Yoseph was sleeping in bed that nightin the house of his masters in the pit because his master had caused him to return to the pit because of his wife. The angel caused him to wake from his sleep and Yoseph got up and stood on his feet and behold an angel of G-d was standing next to him. The angel of G-d spoke with Yoseph and taught him all the languages of man on that night and he called his name Yehoseph. The angel of G-d went from him and Yoseph had returned and he slept in his bed and Yoseph was astonished about the sight that ha had seen." (My own translation)
Here we see how Yoseph got the name Yehoseph and (obviously) another source for this name.
The Mishnat R. Eliezer (Parahsa 17 p. 321-2) states:
שלשה ברחו מן העבירה, ושיתף הקדוש ברוך הוא שמו בשמן, ואלו הן. יוסף, (ובעז), ופלטי בן ליש, ויעל. יוסף ברח מן העבירה, ולפי שלא יאמרו יחידי היה, העיד עליו הקדוש ברוך הוא בעצמו, שנ' עדות ביהוסף. למה נתכנה כאן יהוסף, יה מעיד בו, שלא עבר.
Three people fled from sin, and God combined his name with theirs. They are: Yosef, (Boaz), Palti ben Laish, and Yael. Yosef fled from the sin, and in order that noone should say 'he was alone [and sinned]', God himself testified for his, as it says "testimony with Yehossef". Why is he called Yehossef? God testifies about him, that he did not sin.
Radak (Psalms 81:6), however, seems to downplay the phenomenon noting that this is simply a normal variation that we sometimes find in names:
יהוסף ויוסף כמו יוחנן ויהוחנן, יונדב ויהונדב
Yosef is mitzad olam hazeh, the brothers are mitzad olam habah (the entire Patterns in Time on Chanuka revolves around this theme).
Hashem created olam hazeh with a heh.
Sephas lo yodaati eshma, a malach taught Yosef the 70 languages.
That is Yosef became the nexus of all patterns of living and thus ruled the world.
I.e. Yosef acquired the heh, the expression of God's sovereignty in this world, and so his name is complete.