A friend of mine left his car in front of my house when he went out of town. The car has been here for 1 month and is now being repossessed. I need to know where or who to call to get the car out of here. I heard there was a collection agency I could call or the company where he purchased the vehicle, but I can't find either...

If the car is being repossessed (how do you know this?) then the repo company will come get the car. If they don't know where the car is, if you contact the company, they will come get it. If the car is parked illegally, then call the police.

I wouldn't even worry about it. When the lienholder who's doing the repossession is ready, they'll come and get it. If it's a matter of them knowing your address, just give them a call and tell them. If you're not sure who the lienholder is, go to the source that first informed you that the car was being repossessed.

s_cianci is right. I wouldn't worry about it. If the car is being repossesed, there is little you can do about that. If it has been repo-ed, you could call the police. In Florida when the tow company comes to get a vehicle, they are required to notify the local police, for safety reasons. But it serves another purpose: you call the police to tell them the vehicle is missing and they will let you know if it's repossesed.
But if you're not the legal owner/lein holder, they may not tell you anything.