Megillah, Chapter 4, Mishnah 3. This Mishnah lists various things that may be done only with ten, that is, with a minyan.
The matters that require ten:
"Ein porsin es Shema" - this is an expression that is hard to translate literally, and the simplest explanation has been chosen here: one does not lead the berachos of Krias Shema. One person would recite the berachah, and the others would listen and thereby fulfill their obligation. This is how the Ran explains the phrase; Rashi has a different explanation, which is somewhat more involved.
"Ein ovrin lifnei ha'teivah" - without a minyan one does not step up to the amud to serve as the chazzan and recite the repetition of Shemoneh Esrei.
"Ein nosin es kapeihem" - the kohanim do not raise their hands to bless the Jewish people.
"Ein korin ba'Torah" - one does not read from the Torah scroll.
"V'ein maftirin b'navi" - one does not recite the haftarah from the Prophets.
"Ein osin ma'amad u'moshav" - at a funeral. It was customary to stop seven times on the way to burying the deceased, to sit and weep, to stand and then sit again, and everyone would do so. According to some versions of the text, this was done on the way back from the grave as well. Today only an abbreviated version of the custom remains, and even that is done only with a minyan.
"Ein omrim birkas aveilim v'tanchumei aveilim" - the berachah for mourners is mentioned in the Gemara in Maseches Kesubos: upon returning from accompanying the deceased, they would stand outside, and one person would recite a berachah for the mourners. Tanchumei aveilim - standing in two rows to comfort the mourners, a custom we still practice today. These too require a minyan.
"U'virkas chasanim" - the Sheva Berachos require ten men, and the groom himself can be counted among them.
"V'ein mezamnin b'Shem" - one does not mention Hashem's Name in the zimun. Only when there is a minyan can one say "Elokeinu."
None of these things are done with fewer than ten men.
And for land, nine and a kohen:
The Mishnah adds an interesting halachah: one who consecrated land to the Beis HaMikdash and now wishes to redeem it needs an appraisal by nine and a kohen - ten men, one of whom is a kohen. This halachah is derived from a verse in the Book of Vayikra.
"V'adam kayotzei bahen" - the same applies to a person who consecrates himself, meaning one who declares that his value will be given to hekdesh. Just as with land its worth must be assessed in order to redeem it and give its value to hekdesh, so too with a person: both land and a person are appraised by ten men, one of whom is a kohen.
In summary: This Mishnah lists the things that may be done only with ten - leading the berachos of Shema, going down before the amud, the kohanim's blessing, the Torah reading and the haftarah from the Prophets, the ma'amad u'moshav at a funeral, the berachah for mourners and the comforting of mourners, the berachos for a groom and a zimun with Hashem's Name; and alongside them the appraisal of consecrated land and the appraisal of a person's value, which are done with nine and a kohen.