48 Steps (מ״ח מַעֲלוֹת)

One of the earliest Mussar texts is Pirkei Avot, Ethics of the Fathers (פרקי אבות/PEER-kay ah-VOTE) which is a Tractate in the Mishnah, the earliest Jewish commentary on the Torah. 

Pirkei Avot 6:6 presents a list of 48 Steps. (The Hebrew is מַעֲלוֹת/ma'alot, comes from the verb to ascend עלה/alah, and means steps, stages, or levels.) The list begins by stating that Torah is greater than the priesthood or kingship because these offices are acquired by 24 and 30 steps but the Torah is acquired by 48, which it then goes on to list.

This text doesn't explicitly use the word middah and many of these 48 aren't technically middot as they're quantifiable actions and not abstract qualities, but this list has nonetheless become popular as a list of Mussar middot. (The Mussar Center also offers a separate list of 48 of the most famous Mussar Middot here.)

This list is also available as a printable PDF. 

תַלְמוּד

1.  Talmud   tall-MOOD

Study

שְׁמִיעַת הָאֹזֶן

2.  Shmiat Ha'Ozen   SHMEE-it ha-OWES-enn

Active listening (literally listening of the ear)

עֲרִיכַת שְׂפָתַיִם

3.  Arichat Sfatayim   ARE-ee-chat sfa-TIE-eem

Orderly, articulate speech (literally arranged lips)

בִינַת הַלֵּב

4.  Binat Ha'Lev   bee-NUT ha-LEV

Deep comprehension (literally understanding of the heart)

שִׂכְלוּת הַלֵּב

5.  Sichlut Ha'Lev   seech-LOOT ha-LEV   

Deep perception (literally intelligence of the heart)

אֵימָה

6.   Eimah   ey-MUH

Awe, fear

יִרְאָה

7.   Yirah   year-UH

Reverence, respect

עֲנָוָה

8.   Anavah   ah-nuh-VUH

Humility, modesty

שִׂמְחָה

9.   Simchah   seem-CHAH

Joy, celebration

טָהֳרָה

10.   Taharah   tah-RAH

Purity, cleanliness

שִׁמּוּשׁ חֲכָמִים

11.   Shimush Chachamim   she-MOOSH cha-cha-MEEM

Ministering/being useful to the Sages

דִקְדּוּק חֲבֵרִים

12.   Dikduk Chaverim   deek-DOOK chav-air-EEM

Careful choice of friends; Critical give-and-take with colleagues

פִּלְפּוּל הַתַּלְמִידִים 

13.   Pilpul Ha'Talmidim   PILL-pool huh tall-me-DEEM

Exacting debate with students

יִשּׁוּב בַּמִּקְרָא בַּמִּשְׁנָה

14.   Yishuv Ba'Mikrah Ba'Mishnah   yee-SHOOV bah-meek-RUH bah-meesh-NUH

Calm, clear thinking in Written and Oral Torah (literally seatedness in Scripture and Repetition)

מִעוּט סְחוֹרָה

15.   Miut Skhorah   mee-OOT s-chor-AH

Limited business

מִעוּט דֶּרֶךְ אֶרֶץ

16.   Miut Derech Eretz   mee-OOT DARE-ech AIR-ets

Minimal secular engagement

מִעוּט תַּעֲנוּג

17.   Miut Taanug   mee-OOT tah-NOOG

Limited sexual activity

מִעוּט שֵׁינָה

18.   Miut Shenah   mee-OOT shea-NAH

Minimal sleep

מִעוּט שִׂיחָה

19.   Miut Sichah   me-OOT see-CHAH

Limited chatting

מִעוּט שְׂחוֹק

20.   Miut Skhok   mee-OOT s-chok

Minimal frivolity

אֶרֶךְ אַפַּיִם

21.   Erech Apayim   AIR-ech up-EYE-eem

Patience, slowness to anger

לֵב טוֹב

22.   Lev Tov   lev TOVE

Being positive (literally a good heart)

אֱמוּנַת חֲכָמִים

23.   Emunat Chachamim   emm-oo-NOT cha-cha-MEEM

Faith in the Sages

קַבָּלַת הַיִּסּוּרִין

24.   Kabalat Ha'Yisurin   kah-ball-AHT ha yees-oo-REEN

Acceptance of suffering