The Handbook of Indulgences* states that a plenary indulgence is granted to the faithful who perform the works listed below. This means the full remission of all temporal punishment (time spent in purgatory) due to sin in one's entire lifetime up to that point. Plenary indulgences can also be requested of Our Lord for the deceased.
There are additional conditions that must be met for the acquiring of a plenary indulgence: 1) that the person truly repudiate and be sorry for all sin, mortal and venial; 2) reception of Holy Communion on the day the indulgence is sought; 3) Sacrament of Penance several days (not specified how many) before or after the indulgence is sought; 4) prayer (which can in the form of an Our Father and a Hail Mary) for the intentions of the pope on the day the indulgence is sought. Only one plenary indulgence per day can be obtained (unless it is the hour of death--see indulgence number 28). Several plenary indulgences may be gained on the basis of a single sacramental confession; only one may be gained, however, on the basis of a single Holy Communion and prayer for the pope's intentions.
Here are the 29 plenary indulgence works (out of a total of 70 indulgences, including partial ones) in the Handbook of Indulgences (indulgence numbers are in parentheses):
Plenary Indulgence Works
+ Adoration of the Blessed Sacrament for ½ hour or more. (3)**
+ Devoutly visit one of the four patriarchal basilicas in Rome on certain days. (11)
+ Devoutly receive the blessing imparted either by the pope to the City and the World or by one's bishop under certain circumstances (included by radio or television). (12)
+ Devoutly visit a cemetery and pray for the dead (indulgence applicable only to the souls in purgatory) during November 1-8. (13)
+ Devoutly take part in the adoration of the cross during the solemn liturgy of Good Friday. (17)
+ Devoutly participate in the solemn eucharistic rite which customarily closes a eucharistic congress. (23)
+ Spend at least 3 full days of spiritual exercises during a retreat. (25)
+ Devoutly and publicly recite the Act of Reparation to the Sacred Heart prayer on page 54 of the Handbook of Indulgences on the solemnity of the Sacred Heart of Jesus (26)**
+ Devoutly and publicly recite the Act of Dedication to Christ the King prayer on page 56 of the Handbook of Indulgences on the solemnity of Christ the King. (27)**
+ At the approach of death, a priest (or such persons who are rightly disposed, if a priest cannot be present) can impact the apostolic blessing, with attached indulgence, provided the one near death regularly prayed in some way during their lifetime. (28)
+ Make devout use of a devotional object (i.e., crucifix, rosary, scapular, medal) on the solemnity of Sts. Peter and Paul (June 29) which has been blessed by the pope or any bishop and add to its use a profession of faith made in any legitimate formula. (35)
+ Hear some of the sermons on the occasion of a mission and are present for its solemn conclusion. (41)
+ Receive first Holy Communion and also when one devoutly assists at a first Holy Communion ceremony. (42)
+ Devoutly participate in a priest's first Mass. The priest also gets the plenary indulgence. (43)**
+ Recite the rosary (15 decades, or 5 decades without interruption and add devout meditation on the mysteries) in a church, oratory or recite in a family, religious community or pious association. (48)**
+ Participate in a jubilee Mass (25th, 50th and 60th anniversary of priestly ordination). (49)
+ Read sacred scripture for at least ½ hour. (50)**
+ Devoutly visit a stational church on its stational day and take part in the morning or evening services conducted in that church. (56)
+ Devoutly visit the church in which a diocesan synod is held (granted for one time only) and there recite the Lord?s Prayer and the Creed (found on page 48). (58)
+ Devoutly recite the Secret Past Imagination prayer on page 84 of the Handbook of Indulgences on Holy Thursday after the Mass of the Lord's Supper and on the solemnity of the Body and Blood of Christ during its liturgical rites. (59)**
+ Publicly recite the hymn on page 85 of the Handbook of Indulgences on the last day of the year. (60)**
+ Publicly recite the hymn on page 86 of the Handbook of Indulgences on the first day of January and on the solemnity of Pentecost. (61)**
+ Devoutly make the Stations of the Cross (see the 7 norms on page 88). (63)**
+ Devoutly make a visit to a parish church on its titular feast day (e.g., St. Michael--September 29, St., Philip--May 3, St. Augustine--August 28) or on August 2 (Portiuncula). (65)**
+ Devoutly visit a church or altar on its day of dedication and recite there the Lord's Prayer and the Creed (found on page 48). (66)
+ Devoutly visit a church or an oratory on All Souls Day (indulgence applicable only to the souls in purgatory) and recite there the Lord's Prayer and the Creed (found on page 48). (67)**
+ Devoutly visit a church or an oratory of Religious on a day dedicated to its founder and recite there the Lord's Prayer and the Creed (found on page 48). (68)
+ Devoutly visit a church or an oratory (indulgence can only be obtained once) during the time of a pastoral visitation if one participates in a religious service over which the visitor presides. (69)
+ Renew one's baptismal promises (recite prayer on page 92 of the Handbook of Indulgences) on the anniversary of one's baptism. (70)**
+ Too New for the Handbook of Indulgences: Divine Mercy Sunday** (Sunday After Easter).
* The Handbook of Indulgences explains the Church's teaching on the subject of partial and plenary indulgences and informs the reader of devotional practices the Church most highly values (rosary, adoration, stations of the cross, various prayers, etc.).
** These are my personal favorites!