There are lots of directions one can go in investigating influences on Heidegger or uncovering ideas he appropriated and reworked in Being and Time. Hegel, Kant, Descartes, Kierkegaard, Nietzsche, etc. One of the more interesting might be Ludwig Feuerbach, a post-Hegelian and pre-Marxist who is most well known for a critique of Christianity (and later religion in general) in which he claims that god-hood and religion are simply projections of our own humanity and subjectivity. Worshiping divinity is celebrating human accomplishment.
In countering abstraction and transcendence, Feuerbach turned to a view of human beings as material organisms connected to the world through body and senses, that have a sense of community and a desire for fulfillment which manifests as desires. Ultimately he was still preoccupied with solving a problem of knowledge (epistemology) and altruism (ethics), but you can see this same structure reflected in B&T. Heidegger's move is to treat the question of these 'facts' as ontological - which requires a different way of talking about them.
Another notion that Feuerbach introduces is the alienation of the subject from the community. He says that focus on the subjective "I", misses the fundamental "I-Thou" relationship which part of human existence. Love, which involves an essential I-Thou, is the defining desire of humanity, if you will and death, because it is the one shared experience, represents the last surrender of the self. For Marx, this idea was essentially political (alienated labor) and for Buber, ethical. I don't want to stand too firmly on this assertion, but there is something here that I imagine influenced Heidegger's idea of authenticity. The inauthentic person is someone who is alienated from the world and hence his/her essential humanity.
Feuerbach is a challenging and influential thinker who none of us will probably ever spend much time on, but I was surprised doing a little digging how much he echos in B&T. Check out the canonical entries for some background:
The Wikipedia entry has some biography but is otherwise weak and IEP doesn't even have an entry for him. Alternatively, you can listen to this rap about the Feuerbach neighborhood in Stuttgart.