T'ka b'Shofar Gadol L'cheruteinu
I would like to add to Jason's post below. We say three times a day: "Teka b'shofar gadol l'cheruteinu, v'sa nesh l'kabetz galuyoutenu, v'kab'tzenu yachad me'arba kanfot haaretz (l'artzenu)" - Sound the great shofar to announce our redemption, and raise a banner for the ingathering of our exiles, and gather us together from the four corners of the earth (to our land). (Nusach Ashkenaz leaves out "l'artzeinu" - to our land. I say this word anyway, because we must be clear about the destination of kibutz galuyot!)
Why is it the shofar which will announce our return to Israel? The main function of the shofar, as Jason said, is to call us to repent. It is a wake-up siren, waking us from our sleep to a state where we are conscious of our sins. Sometimes, when I wake up, I don't remember where I am, or what time of day it is. It takes a few minutes before I am fully aware. This is the case on the national level as well.
When we in the exile wake up this Rosh Hashana, we may feel like we are at home. But the shofar calls to us to remind us to wake up fully, and realize where we are - we are in exile, that is, not home, not where we are supposed to be. We are not in exile because we have been forced here; we are in exile because we've chosen not to return to the land which God has given us. We can only make this choice if we are not fully awake- if we are not conscious of our exile.
May the shofar this Rosh Hashana awake in us a new consciousness, to lead us to a full ingathering of the exiles to our land!
K'tiva v'Chatima Tova!