An enormous number of recognized saints had (and have) an intense devotion to Mary, but certain ones were especially known for their dedication to her, being inspired by her, even being contacted by her. Here are some of them:
Francis of Assisi
Gertrude of Helfta
John Lydgate
Ignatius of Loyola
Margaret Mary Alacoque
Juan Diego
Bernadette Soubirous
The link below is to the text of Gerard Manley Hopkins' poem, put online by Father Stephen, an Orthodox priest:
Prayers for May contains links to individual prayers devoted to Mary. From About.com.
For devotional books in LINUS and LINK+, use the Subject Heading:
Mary, Blessed Virgin, Saint - Meditations
For information about feasts, titles, iconography and more see:
The Mary Page (Univ. of Dayton)
For Latin lyrics of some major hymns to Mary, with English translations, see:
To the Virgin Mary (Schola Cantorum Bogotensis)
The Annunciation
Image in the public domain: from Wikipedia Commons