Actually the future subjunctive is not dead yet! (I bet no one learned it in an American high school though.) Although many Spanish-speakers will never use it, most of them will have seen it in the following situations:
  • Most commonly, in the literature of the Golden Age, including in the works of authors such as Miguel de Cervantes and Lope de Vega. (It is used either after si where today we would use the present indicative, or often in a construction with a future indicative, where today we would use the present subjunctive instead.)

  • In proverbs; e.g. Si a Roma fueres, haz como vieres. (When in Rome, do as the Romans do).

  • In expressions; e.g. venga lo que viniere . . . (come what may . . . )

  • In legalese, used to refer to an indefinite person; el que hubiere reunido mayoría absoluta de votos será proclamado Presidente de la República (the one who has received an absolute majority of votes will be proclaimed president of the Republic)

  • In especially poetic or flowery writing, or in really flowery speech.

  • In certain translations of the Bible; Y dijo: De cierto os digo, que si no os volviereis, y fuereis como niños, no entraréis en el reino de los cielos (Mateo 18:3, Reina-Valera). "And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven" (Matthew 18:3, Revised Standard Version).

  • In journalistic prose; I hear that Argentinians are especially fond of this.

The future subjunctive tense is easy to form; simply take the imperfect subjunctive tense and change the "a"s to "e"s. Some examples:
For the verb hablar: hablare, hablares, hablare, habláremos, hablareis, hablaren.
For the verb comer: comiere, comieres, comiere, comiéremos, comiereis, comieren.
For the verb salir: saliere, salieres, saliere, saliéremos, saliereis, salieren.
For the verb tener: tuviere, tuvieres, tuviere, tuviéremos, tuviereis, tuvieren.