Attack the Block
Of course, it helps that they possibly instigated it, after they slaughter a lone alien that lands on a car next to them. After that, more show up and they seem to be out for blood, or at least that of the kids that destroyed their brethren. In the meantime, they're just trying to survive and make sure that the residents of their building don't get eaten.
It's a horror/comedy hybrid, though I would say we're looking at about 80% horror and 20% comedy. The cast is likeable enough for a bunch of thugs, and although you dislike them at first, they do grow on you. I really liked the design for the monsters too. It's very unique and works in a less is more kind of way. Cheap, but effective. Good gore too. Quite a few scenes have us surprised by people getting killed when we least expect it, and I like it that way. It keeps you guessing as to who is safe or not in the movie.
A fun film and well worth a viewing or two. Check it out.