Hymns to Lord Vishnu are found throughout Vedic, Puranic, and Epic literature. From the Rgveda comes the Vishnu Sukta, which describes Lord Vishnu’s three strides across all the universe, and the assertion that Lord Vishnu’s abode is eternal. Also from the rgveda is the famous Purusa Sukta, referenced by the other posters below, which is repeated in the other three Vedas as well. This hymn extols Vishnu as the material cause of creation. Additionally, the Taittiriya Shakha of the Yajurveda contains the Narayana sukta, which emphasizes the all pervasiveness of Vishnu, accessible to yogi who meditated on the divine presence within himself. Other hymns addressing Lord Vishnu are found in abundance throughout the Veda. One interpretation of the Gayatri mantra, for instance, is that each of its 24 syllables represent a form of Vishnu. For a Vaishnava, in fact, the entire Veda praises Vishnu alone, as he is the in dweller of all the deities Vedic hymns address (although other sampradayas differ on this point)
The Ramayana is treated as a means to propitiate Lord Vishnu in His avatara as Lord Rama. In fact, in south India, repeated recitation of Valmiki Ramayana, especially the Sundarakanda, is an integral part of many family traditions.
Referenced by every other poster, and for good reason, is the Vishnu Sahasranama. This is the celebrated hymn of the thousand names of Vishnu. Embedded in the Mahabharata, it is praised in various Puranas, by great saints, and is honored even in some South Indian saiva literature. In days gone past, someone desiring Moksha had to walk an ascetic path exceedingly difficult; one might have had to perform fire sacrifices twice a day, memorize an entire Vedic Shaka, fast and completely abstain from wordly pleasure – and that too only with a pure mind. The claim of the Vishnu Sahasranama is that one who chants those thousand names with faith and love has that hard gotten moksha in his very reach. Not only is the stotra a means to call on the Great God, but it is a meditation on the Supreme itself.
One cannot forget the vernacular literature on Lord Vishnu. Some works that are still influential today are the Tamil Divyaprabandham, the collected hymns of the Alvars; the Kannada songs of the Madhva haridasas; the lyrics of Mirabai and Surdas in praise of Lord Krishna; and the Ramcharitmanas of Tulasidasa, a medieval Hindi retelling of the Ramayana. Many of these works are so highly regarded by those who study them that they are cal