Lama Tsong Khapa's influence in Tibetan Buddhism is undeniable. Much Buddhist thought since he founded the Gelugpa sect is a response to him. The Dalai Lama himself, generally one of the most influential Tibetans, is of the Gelugpa sect. But Tsong Khapa never placed himself higher than others, preferring to be seen as an ordinary monk. Even though he could have used miracle powers such as clairvoyance, he insisted that such powers not be displayed.
Tsong Khapa's most important achievements include uniting Sutra and Tantra teachings, and expounding on the graduated stages of the path which were brought to light by Lama Atisha in his Lamp on the Path. Tsong Khapa's Lam Rim Chenmo is an invaluable text which covers the whole path to enlightenment from beginning to end. This all-inclusiveness is why it is seen as such an important work. Another work is the "Lines of Experience."
Tsong Khapa himself was an emanation of the Buddha Manjushri, and was considered to be a Second Buddha and unequaled teacher. The great Pabonka Dorje Chang had these words to say about the special relationship between Tsong Khapa and Buddha Manjushri:
Manjushri appeared to Je Rinpoche [Tsong Khapa] all the time, and they had a guru-disciple relationship. Manjushri gave Tsongkhapa an infinite amount of profound Dharma on sutra and tantra. This can be found in Je Rinpoche's biography. One might therefore claim that Je Rinpoche received the lamrim from Manjushri, but Je Rinpoche himself concentrated on tracing the lineage back to our Teacher Shakyamuni [the historical Buddha]. He never said it came from any vision, or any more recent lineage, and so on. This is one of the wonders of his biography.
— From Liberation in the Palm of Your Hand by Pabonka Rinpoche
From childhood he practiced three of the most powerful tantric practices (Hevajra, Heruka Chakrasamvara, and Yamantaka). But he always stressed that one should never forget or ignore the teachings of Vinaya, hinayana, and mahayana, even as an advanced practitioner. It's no secret that the tantric path allows things like eating meat, which is not allowed in the mahayana path. But though the paths seem different, they are not really contradictory or in a sense "stratified," like the various traditional castes in India, as the differences between them are simply an expression of their emptiness, not a denial or self-negation. In other words, the paths are in a reciprocal dependent relation to the practitioner, and to one another. Thus a tantric practitioner will still respect animals, though he or she may eat them once in a while.
Some important practices relating to Tsong Khapa which we use in our Sangha are the prayer of "The Stages of the Path," Guru Yoga, the Mantra of Lama Tsong Khapa, and the prayer of "The Hundreds of Deities of the Land of Joy."
Compiled from the following sources: Wikipedia, The Stanford Encyclopedia of Philosophy, the Kadampa Buddhism website, and Liberation in the Palm of Your Hand, edited by Trijang Rinpoche, and translated into English by Michael Richards.