I don't rent, but I live in town. I went through NIU about 7 years ago.
From what I can recollect, you do NOT want to rent from Mason Properties if you can help it.
Other than that, it depends what you might be looking for-- if you just want a room in a house and you don't care if you know your 6 or 7 other housemates, then you can usually find a room. Dan McClure is a good resource-- although he is not a perfect landlord, he isn't half bad either.
There are some nice apartments on College Avenue, and then some others over on
The university also does run graduate student housing, which I am largely unaware of, but do know that it is "apartment-style" living.
Stadium View Apartments used to be nice, but is right outside the football stadium, so things can get crowded and noisy on football days. Not much of an issue if you are a football person.
Suburban Apartments and Estates is also fairly well run, although I believe they have a strict no-pets policy if that is an issue for you. There are a good mix of "townies" and students there.
I hope that helps! I am a bit out of touch, but I saw no one had responded to you, so I thought I throw in what I know.