Lower voiced women in opera are usually mezzo-sopranos, but occasionally one hears a true contralto, the lowest female Fach. Today I know of only two contraltos: Ewa Podleś and Meredith Arwady. Mezzos and contraltos will need notes well below middle C, and a mezzo should have a high B flat at least. I'm not sure there is a sharp line separating mezzo-soprano from contralto, but I will try. Here is a list of the sub-categories for mezzo-sopranos and contraltos.
I am now going to describe the sub-categories, but please be aware that the same singer may show up in different sub-categories. A role may also cross into more than one category. I have tried in selecting these examples to make sure that the singer is actually of the suggested sub-category.
The coloratura mezzo is primarily a phenomenon of the Baroque and bel canto. Sometimes the male hero is in this Fach, but most of our examples are female roles. Angelina in La Cenerentola (Rossini), Romeo in I Capuleti e i Montecchi (Bellini), Costanza in Griselda (Vivaldi) [Wikipedia lists Griselda as a coloratura mezzo when she is barely a singing role at all. Costanza has all the good singing.], Rosina in The Barber of Seville (Rossini), etc. This example is Cecilia Bartoli singing "Non piu mesta" from La Cenerentola.
And only La Bartoli runs to the top of the cake.
When the tenor gets heavier after Rossini, the mezzo follows. This voice generally does not perform coloratura like the mezzo in the previous Fach. Charlotte in Werther (Massenet), Carmen in Carmen (Bizet), Nicklausse in The Tales of Hoffmann (Offenbach), Octavian in Der Rosenkavalier (R Strauss), etc. Here Elina Garanča sings the Habanera from Carmen.
This begins with Verdi and is at the heavy end of operatic repertoire for the female voice. In German repertoire there is never coloratura, but in Italian repertoire, especially Verdi, there might be. Eboli in Don Carlos (Verdi), Azucena in Il trovatore (Verdi), Marina in Boris Godunov (Mussorgsky), Ortrud in Lohengrin (Wagner), etc. Our Italian example is Fiorenza Cossotto singing "O Don Fatale" from Don Carlo.
Our German example is Christa Ludwig singing "Entweihte! Götter!" from Lohengrin.
Since there aren't ever enough contraltos to go around, anything designated for a contralto will often be sung by a mezzo-soprano. I will attempt to find examples of true contraltos who are known for the extreme darkness in their voices. Roles for coloratura contralto include Isabella in L'italiana in Algeri (Rossini), Tancredi in Tancredi (Rossini), etc. Ewa Podleś sings "Di tanti palpiti" from Tancredi.
I only know of one. We will suggest others. Here is Marijana Mijanović singing "Empio, Dirò, Tu Sei" from Giulio Cesare.
And this is Marie-Nicole Lemieux singing "Nel profonfo" from Vivaldi's Orlando furioso.
And then there's Ewa Podleś singing "Pour Une Femme De Mon Nom" from Donizetti's 1840 La Fille Du Régiment. We keep coming back to her.
This is the same dark voice in more legato music with virtually no coloratura. Mother in The Consul (Menotti), Pauline in The Queen of Spades [Pique Dame] (Tchaikovsky), Maddalena in Rigoletto (Verdi), La Cieca in La Gioconda (Ponchielli), etc. Here Olga Borodina sings Pauline's aria from Pique Dame.
The same restrictions as for mezzos apply: Wagner will never show coloratura but Verdi often will, though it has a much heavier sound than for a soprano. Examples are Azucena in Il trovatore (Verdi), Ulrica in Un ballo in maschera (Verdi), Erda, Das Rheingold, Siegfried (Wagner), etc. Hanna Schwarz sings Erda from Das Rheingold.
This is Elena Obraztsova singing Ulrica's aria from Un Ballo in Maschera.
My film selections are based on the quality of the performance and the sound of the voice and often do not include subtitles in English.
Read about countertenors here, sopranos here, tenors here, and here for baritones and basses.